Could also have "spells" that take a "daily" slot, but last all day, and provide more limited actions, as a way to handle "at-wills" or other such spells.
For example, if you have the right feat investment, you can maybe turn a traditional 3rd level slot (i.e. appropriate for a 5th level wizard) into a spell that, once cast, gives you around seven, 1d6+mod magic missile spells, as a resource you can use anytime during the day.
This is similar to the old magic missile that gave multiple missiles per casting at higher levels, except you get more of them to make up for the fact that you can't use them all at once. So the traditional Vancian "spell economy" is used to set your major limits by "slot," but what the "spells" in those slots do is widened considerably in scope and mechanics.
I'm sure people could come up with other interesting structures besides turning said daily into a quasi-at-will, too. Once there is a layer of indirection between the mechanics of the "spell slot" and the in-game "spell" that ultimately comes out of it, the relationship doesn't need to be as strict as the traditional model made it.
You could also then reuse the mechanical framework but not the in-game output for some different things for non-magical stuff. For example, instead of "exploits," a fighter has "practices" that govern where he is currently honing his techniques. He slots these "practices" similar to 4E, but what they enable are some "encounter", "at-wills," 3E barbarian rage-style bits, or even some straight, reusable abilities, almost like feats that can be retrained within certain limits between each adventure.