If it makes the game easier to DM.
If it gives fighters fun things to do at high levels.
If it gives bards fun things to do, period.
Heck--if it gives everyone more fun things to do.
If it makes combat faster.
If it makes combat more interesting--more tactical options. Especially, if it makes the tactical game more obvious, because that's a part of the game that doesn't always come naturally, but is always fun if I can come up with something.
If it makes high-level play functional.
If it makes single-monster encounters less horribly swingy.
If it really does achieve the goals that Mike Mearls outlined in the September podcast--fix the math, fix the class balance, fix the encounters-per-day boondoggle. Note that none of these problems make 3rd unplayable. But fixing them would make it a lot more playable, and a lot more fun.
If they make non-combat interactions more interesting, more dynamic, and more relevant.
If it's got cool flavor. If the flavor in the base books is cooler than the flavor in the base
If it's easier to run encounters with multiple monsters with very different abilities.
If it's easier to build fun monsters, and otherwise work with the system. Particularly, if the design and development ideas are more transparent, because it's easier to mess with things if I know why they're a particular way in the first place.
If multiclassing is really as good as they say. "Any combo, any level, always works," and I'm totally there.
If turning undead is made less stupid.