1. Adventures work on your senses and imagination. What is pleasing? Can a puzzle be pleasant? 

I like that, when done right, adventure games can provide a story/experience that is both directly presented to you via writing, visuals and audio while also leaving room for your imagination to fill in the blanks. I think of them as interactive books really.

Puzzles that provide a challenge without becoming frustrating can be pleasant too. Figuring out a solution to a hard puzzle in your head by thinking things through logically while doing other things then later returning to the game to find it's the right answer is the best.

2. Role immersion. How does a game make you want to engage with its world - and how does it fullfill that desire? What makes you feel content playing a game?

Interesting premises and unique player characters help immersion. Having these characteristics bleed through into the writing, actions a character can do and the solution to puzzles really adds to that too, for example Ben from Full Throttle having a dedicated kick action and using brawn as a solution to live up to his outlaw biker character was refreshing when compared to the far more common geeky leads of adventure games at the time.

3. Gratification. What makes you feel like your actions pay off? Is a good puzzle difficult - and what makes it so? Are all forms of difficulty equally gratifying? Does it matter how a puzzle is embedded in the game? And are there other gameplay elements in AGs that can give you this feeling of accomplishment and reward?

I would say a good puzzle is one that can be solved without frustrating the player enough to require hints or a walkthrough. While nothing can universally be perfectly designed for every player on the planet, if many people get completely stuck and needs hints/walkthroughs often at the same part(s) of a game then it points towards a fault of the design of that game. That does not mean all puzzles should be easy though, in fact far from it as solving a hard puzzle you've been at for a while is very satisfying.

I think part of the problem is the uneven puzzle difficulty many adventure games have, and perhaps some lessons could be learnt from dedicated puzzle games that aim to progressively build on 'aha' moments could be worked into the genre more. Good puzzle difficulty progression can be seen in games like The Witness which, if I remember correctly from listening to interviews/podcasts, the creator has actually mentioned was directly inspired by Myst and his frustrations with the puzzles despite liking it in principle. In that game it introduces concepts and rules in initially simple puzzles that naturally build into larger more complicated solutions that combine them as the game progresses.

I also dislike embedded hint systems, hints should be naturally worked into the game rather than something bolted on.

What really hooked me on the genre, and I don't find it as much lately, was the feeling of playing in an open world, which was a crazy concept in the early 1990s. Walking around Melée and discovering more and more screens, with streets, alleyways, buildings you could enter, intriguing characters... and then you could walk the opposite way, into the forest, and you could get lost in it, and find a circus, and...

All that right from the start! You didn't have to solve a single puzzle to explore that big open world.

I had the same feeling in KQ1 with its giant map. I didn't care how hard the game was. The sense of exploration was everything to me at the time.

Oh wow, I love these answers! Lots of great comments about puzzles, genuinely excited to see it. I'd like to share some thoughts but for now I hope more people chime in.

Happy to see exploration and discovery get some recognition as well.

And some of the answers are really funny - here's looking at you Jen. 

@BehindTimes: text adventures are great, I think you can get away smuggling a couple into a thread here and there. If there's enough interest I think we could pull off a text adventure CPT.

One thing that's already sorta been said that I like about adventure games is that you get to appreciate the graphics more.   There are so many games now with great graphics but when your busy shooting or killing stuff or trying to stay alive you don't really appreciate or look at the graphics as much. I wish games were were a bit slower paced and didn't throw so many enemies at you.

I have played and loved many third person games, but my heart beats a little faster when I have a first person game to play. There’s something about looking at a scene through my own eyes that pulls me in and doesn’t let go. I’ve never minded that I don’t exactly know what to do next. I’m a slow player and prefer to wander, seeing what I can manipulate, read, confiscate, and solve while learning about the world around me.

Puzzles and story are important in most games, but the puzzles should be integrated into the world I’m in, and the story can be told through characters, books or journals. That being said, I do love puzzle-heavy games, and I especially appreciate clues to puzzles that are given in unique ways.

I also like being wowed by beautiful graphics and a great soundtrack, but they aren’t necessary for me to enjoy a game.

Maybe I'm too easy to please, but all this boils down to the fact that, so far, I have not played an adventure game that I didn’t appreciate in some way.

Great contributions.

I like the notion of adventure games giving you a chance to explore a world at your own pace. It's like you're reading a novel in a different form. I also like to take my time. Look at a painting, examine a plant, talk to a patron at the bar. That's pleasant to me.

The first graphic adventures I saw made a huge impression, but a well-written text adventure can be equally beautiful, conjuring up all sorts of scenes in your imagination. I think what makes me want to hang around in a world is tone and atmosphere. It helps if these are coherent, though it can be funny or interesting if they're not. The world has to make it sense, even if it the rules and expectations seems alien at first.

Interesting thought about first person games. I'll keep that in mind, maybe more people would appreciate a community playthrough of one of 'em.

I like all sorts of puzzles. I'm cool with games that are created around a set of puzzles. I think the amount of challenge does increase the sense of accomplishment. That said, a creatively designed puzzle can be enjoyable in different ways. It can be part of the dynamic between characters. Maybe it teaches you something about the world - or it's an example of something you learned before. That's more about engagement than doing the most mental gymnastics.

A puzzle can also just be beautifully integrated into the world. It can be gracefully designed. In that way I think it can be pleasant.

1) To me a "puzzle" is only pleasant if it feels like problem-solving rather than hoop-jumping. I don't want solutions to always be as simple as "use hammer on nail," but if it's not organically intuitive within the context of the game (the crazier the game, the greater the latitude), then it's nothing but an arbitrary roadblock. Ugh. (I frequently confess to sucking at puzzles, but really it's only the latter I'm bad it, as much because I don't care about them as not really vibing with their horrible design.)

Also, puzzle clues should be proactively layered, anticipating the player experience. Not just "here's your clue; you get it or you don't," but rather "if you're doing X instead of Y, then perhaps there's a different approach." A hint system that isn't a hint system, based on anticipated failure.

2) A game should have its own unique vision, even if it has a familiar setting. There's got to be something distinctive to THAT game world to inspire me to explore. I bore easily so no excess info dumps or extended conversations. A good mix of necessary and optional hotspots to examine is welcome, but NO "wasted" interactions. If it's not necessary AND not funny/insightful to building the world or fleshing out the story and characters, then it's just narrative clutter.

3) Not only does a puzzle need to be well-integrated, solving one MUST result in a substantial new area to explore or other new juicy bit of gameplay of some kind. I've lost count of how many games bottleneck with literally one puzzle after another. A dev can't (and shouldn't) entirely control pacing, but can control the immediate reward for overcoming obstacles.

A paradigmatic example that has it all -- which is barely an adventure game in the traditional sense -- is Papers, Please.

1. I'm not sure I would call the game or the puzzles "pleasing," exactly, but once you understand what the game is asking you do to it is very satisfying.

2. This is where PP shines in a way that very few other games have been able to reproduce. Almost all games purport to do this to some degree, but it's usually somewhat artificial. For example, in King's Quest, yes, you are a king, but are you really doing king-like things?

3. Again, PP accomplished something there that is rare. The game causes you to actually taste the experience the ups and downs of literally doing a boring, mundane job. No slider or inventory puzzle will ever accomplish this.

The common thread is that PP gamifies a slice of real life and turns it into a compelling story. Some other examples that come to mind are Obra Dinn, Orwell, Valiant Hearts, and This War of Mine. These aren't my favorite games, but they do come closest to capturing what I'm looking for in my ideal game.

I think the thing I enjoy the most about adventure games is that they (typically) allow for time to think. Now, I can get decently good in other games that require you to do things within time limits or have quick reactions, and I can enjoy them too. But that does come with some amount of (intentional and mostly subconscious) stress. Being able to sit down with a game that doesn't create that type of stress, while being able to also solve puzzles and get a good story, is just very appealing to me.

1. Adventures work on your senses and imagination. What is pleasing? Can a puzzle be pleasant?
A puzzle can be pleasant for me when it is designed in a way that it makes sense in the context and narrative of the game. I also like puzzles where you manipulate objects to unlock a new level and get nice instant visual and audio feedback, when it "clicks" if I do it right. I enjoy responsive puzzles and of course it is enjoyable when a puzzle is successfully solved and I managed to do this on my own without help/cheating.
I want to enjoy the solving already while I am solving it though, and not just enjoy "having solved" a hard and complex puzzle after it is done.

2. Role immersion. How does a game make you want to engage with its world - and how does it fullfill that desire? What makes you feel content playing a game?
I don't remember having played an adventure game where I really felt in the role of the character I played. I am playing roleplaying games and have played pen & paper roleplaying games, so this is what I'm comparing with. And it's just different when the given character already has a name and certain traits, background and motivation. But still: It helps when the fate of my player character is something I can feel and understand, so I want to "help" them to reach their goal. 
A game makes me want to engage with the world when there are details I can actually look and interact with - and when I get feedback/a response when trying this. Getting unique (instead of generic) feedback that makes sense and is sometimes surprising (not just my character describing what I already see anyway with my own eyes), I am happy.

3. Gratification. What makes you feel like your actions pay off? Is a good puzzle difficult - and what makes it so? Are all forms of difficulty equally gratifying? Does it matter how a puzzle is embedded in the game? And are there other gameplay elements in AGs that can give you this feeling of accomplishment and reward? 

Hm...gratification for me means finding out things about the world and the story I am exploring. It can be a good written dialogue, earning true answers by helping npcs, or solving a puzzle to open a door to a new area. Making the world of the game a bit better if I can.