[example story:]

I think that good adventure game puzzles give the player the illusion of freedom.  Example:  allowing the player to pick up a rock and throw it at the snake, causing the snake to bite you and you die.  This way the player has exhausted all the routes that made sense to them, but still received a reaction that made equal sense.

Indeed. But Kings Quest, Space Quest, ... already implement such stuff, though not nearly enough.

and here's my point:

2d, 2.5d or 3d: doesn't matter, if it's a good game it won't bother anyone a bit.

'not enough', that's the feeling you get when playing most adventure games.

most puzzles, interactions, ... are most of the time limited to advance the plot, and nothing more.

an adventure game should be far more!

-> add new locations that serve no actual purpose for the main plot, but that people have been dying to explore

(I remember KQ5 and the wagon blocking the road to the rest of the town. I searched for hours to try to find a way to get the wagon off the road, or at least be able to pass it.)

-> add a bunch of subplots next to the main plot, totally optional to do, serving NO advancement whatsoever

...then allow some of the stuff acquired in these subplots to be used to solve other subplots, and possibly even give an alternative solution to the main plot at times.

-> alternative solutions, another thing: why must there be only 1 way to do something? granted, multiple solutions have been done, but not nearly as much as I'd like.

note: these alternative solutions shouldn't be obvious, either. They should be as difficult to come up with as the original solution;

next,

-> story: rather then setting the goal in advance (like 'save the princess', then end the game by 'saving the princess'),

don't tell the player what's gonna happen or what needs to be done at the end. That should be a surprise!

rather have a bunch of subgoals, which either succeed or fail (doesn't matter which), and after having reached each subgoal, the player character is forced into yet another quest, to solve this or that, untill eventually you get to the last goal, which then wraps up the entire story with several possible alternative endings...

example story:

part 1:

a farmer starts his day on the farm. original task given to him: do the work at the farm.

then:

- first option: solve all the 'working-at-the-farm' puzzles'
- second option: decide to slack off, find a place to rest (like a haystack)
- third option: go bother the witch in the woods for a love potion to win over your sweetheart.

then, no matter what you've done, a big giant meteor falls out of the sky, leveling the farm.

part 2: "what the -"

the farmer survived. but what to do now?

no clues should be given to the next goal. rather, have the player try out stuff.

- option 1: go to the remains of the farm, see what you can find out
- option 2: get the hell out of there, run to town to get help
- option 3: ignore all above, and go rest in the haystack. let the local wizard(s) deal with this one.

then, depending on what you've decided to do, the plot advances.

the farmer could end up defeating an evil creature, or winning his true love, or even just getting his farm rebuilt. or perhaps he just gets a new job+home in town.

anything's possible

note: there can be more options then the 3 I used in this example, of course.

now that's the type of game I'd like to see.