I'm pretty sure I've said this elsewhere in the forums, but what I really appreciate in adventure games are puzzles that are something other than "find object X, go to place Y, use X to Z". Flight of the Amazon Queen offers some of my favorite examples - there are quite a few puzzles that are solved through dialogue, including at least one instance of going down a conversation path, failing to accomplish the goal but learning something vital, then returning down the same path and using that information to get where you need to go. Other puzzles involve reusing the same item in multiple, clever ways - there's a vacuum cleaner that serves a number of purposes, and a knife that's handy almost any time you need to cut something. I'm not talking about the Myst-style puzzles where you toss the inventory aside and just push buttons until something happens, but things that don't make you feel like you're trudging down an ordered list of events that lead to a conclusion. Retreading old ground in a new situation can be fascinating if done properly, but it rarely is. Remember finally navigating the cave maze in the first Legend of Kyrandia, getting the Will-o-wisp spell that made moving around the cave trivial, and realizing there was no point to it anymore? It would be nice if some of the objectives could wait until after you had the spell so there was a reason to continue exploring with it. Lambo's suggestion is a similar element, where you put together information to come to a conclusion that solves a puzzle. If there's a lot of reading, though, there has to be SOMETHING to make it not boring. Voiceovers like Alone in the Dark are one possible idea, for ironic effect.
Basically, I love using inventory items to solve puzzles, but I hate feeling like an item exists just to be the solution to a puzzle. It makes the game feel contrived.