Nonstacking.
Lets say that Trip in 3e is a station wagon. In 3e, you own this station wagon, and perhaps you want to improve it. So you spend a feat to get rims. Then you spend a feat to get hydraulics. Then you spend a feat to get neon lights. Every feat you spend upgrades the same station wagon. When the core rules come out, maybe the only thing you can get for your station wagon are rims, and the game is balanced on this assumption, but eventually the designers create all kinds of things for your station wagon, and you end up with a massively pimped out ride that isn't appropriate for the power curve that you saw in the core rules.
In 4e, something like Trip isn't a station wagon. Its like a brand of station wagons. If you want the station wagon with the sweet rims, you choose the power that gets it for you. If you want the one with hydraulics, you choose a different power. If you want both, you obtain two different station wagons, and choose which one you want to ride at any given moment. And if you want a station wagon with rims AND hydraulics, you wait until you're level is the appropriate one, and then pick it.
Meanwhile, you can spend feats to improve your Trip abilities, and the feats you spend will improve ALL of your trip abilities, but feats aren't as amazing- they're little things like fuzzy dice for all your station wagons, where powers are big things like low-ride modifications to just one.
This makes power creep MUCH more manageable. It will still happen to a certain degree, because more feats always means more optimization as you combine them to craft exactly what you want, but this change will make a major difference in keeping things under control.