A couple of things that make me think of Lord Of The Rings, originally posted in honor of the Baggins boys' birthday...
LIKE: A REASON TO GO ON THE ADVENTURE
Maybe it's a pack of dwarves on your doorstep. Maybe it's a family heirloom with a sinister past. Whatever it is, it's a lot more fun than just walking into a dungeon because your GM told you that's where you were going.
This is especially important to me when I'm the one running the game. It's important to me that the PCs have good reasons to do whatever they're about to do - revenge, curiosity, obligation, etc. It's best when these things come from the characters themselves; my favorite adventures are the ones that come from the characters' ambitions. Failing that, I like having a framing device - an organization that the PCs belong to (such as a superteam), or perhaps a collection of nine brave souls who are walking from here to the big volcano.
As is often the case, this is about immersion. I find it much easier to suspend my disbelief if the adventure grows organically from things that have come before. Shadows from the past, if you will.
DISLIKE; SPLITTING UP
Seriously, people. Don't split up. Stick together.
My "a-ha" moment with this came a couple years ago, after I'd taught my friend Scott to roleplay. Scott made it into his late 30s without ever having tried RPGs, but the news that there was a Star Wars game tipped him into our camp. So we played a few sessions, and then I asked him what he thought about the various tropes and expectations of roleplaying. Turns out he really hated it when the party split up. "It's so boring," he said.
And he's right. It's boring. So when I run games now, I ask the players to not split up unless they really need to. In return, I create adventures that don't require so much splitting up - I design my encounters with the assumption that everyone will be involved. What we lose in reality-simulation, we more than make up for in fun-having. This isn't a novel, after all. We aren't being entertained by Gandalf and Pippin in one place, then Merry in another, then Frodo and Sam in yet another. This is a group activity - so don't sideline each other without a good reason. Don't split up unless you have to - and expect that we'll resolve your solo activities very quickly. Because while one or two players are having fun, everyone else will be bored.