I remember seeing a quote implying that first level characters to not have as many powers as higher level characters, but that at a certain point, the amount you have plateaus out.
So I assume characters get "new" abilities for a couple of levels (presumably the "utility" abilities), and after that you swap out your powers for new ones. (except possibly the Wizard, who gets new ones to prepare from).
One would assume you don't get too many more encounter and daily powers, or the "action economy" ceases to be an economy.