Bleah.
... not bothering with trip mechanics at all. 4e's approach as explained by Hussar is better, but still far too mechanical. Why not just let the DM wing it on those infrequent occasions someone actually tries to trip someone (as opposed to just knock 'em flat) during combat?
Lan-"trippy, man"-efan
Well, the 4e answer would be that movement in combat is REALLY important. I know people don't like the minis centric focus of 4e combat, but, it's a basic design decision. AD&D for example, didn't really focus too much on tactical level movement. Yes, there was stuff there, but, it wasn't a huge deal.
4e focuses very strongly on tactical (read combat) level movement. It's not unusual for a character to move every round, and quite possibly several times per round with all the push/pull/slide stuff going on. It's just another way to engage the players during play.
RC - Oh totally. But, when you have mechanics that are based on the premise that these are modeling the physics of the setting, those mechanics are going to dictate to a large extent how this game is going to be played. Whenever you say, mechanically, that X works like
this then that is going to have a huge effect on how play goes at the table.
The thing is, I don't think the 3e trip mechanics are actually that bad. They do the job and work pretty well.
But, at the end of the day, whether you model it before play or you simply leave it up to the players to model it in play, the point is, the mechanics still have a very strong effect on how the players interact with the game world.
And, going back to Lanefan's point about leaving it entirely up to the group, IME, what happens is that these things are just ignored. How often did someone try to trip someone in 2e? I don't know about your group, but I can't think of a single instance that it occurred.
Simply free-forming isn't really an answer. Games lacking particular mechanical resolutions tend to produce games where that element is simply ignored. Games which have some guidance as to how to resolve elements tend to direct play towards those things.
At least in my experience. Obviously YMMV and all that.