From the WotC Rule of Three R&D column (bolded and underlined parts mine):
Question: What's the single biggest lesson you've learned about D&D's design and development since the start of 4e, and how are you applying that info?
Answer: I chatted with some of my coworkers, and opinions vary. But here's one that I think about a lot, and my colleagues generally agree with me: We have too many powers that are too similar. Listing powers under specific classes might have helped organize the Player's Handbook for the specific task of character creation, but it launched us on a design and development path where we created many similar powers whose only substantive difference is the class those powers appear under. If I told you "I'm thinking of a 2[W] power that dazes for 1 round—which class does that power belong to?" you couldn't begin to guess. Almost anybody might have that power.I've been seriously disappointed with Monte Cook's Legends & Lore column series, but this one makes me optimistic that the R&D department is seriously thinking about their game in a good and productive way, with serious insights.
In earlier editions, some spells were allowed to appear on multiple class lists. We considered this a moderate nuisance in 3rd Edition, because it was strange that you couldn't describe hold person as a 2nd-level spell—for the wizard, it wasn't. I have belatedly come to realize that overlapping spell lists are a good thing, because they give spells like hold person anddispel magic unique identities in the game. When I play 4e, I don't recognize most of the powers that my fellow players are using, and that's a shame. In retrospect, I wish we'd just created a Powers Appendix of iconic, diverse effects (including martial powers, of course), and granted each class access to different subsets of those powers. The game would be better with a smaller number of iconic and memorable powers even if classes overlapped a bit more.
That ship's sailed, but we are looking at ways to be more conservative with the creation of new powers (or classes requiring entire power sets) in the current environment. Certainly the Essentials versions of the fighter, rogue, and ranger offered different ways to play functional characters with fewer powers. Introducing builds instead of classes is another way to create greater overlap in power selection. Upcoming products showcase more examples of both these approaches, which we now think are a little better for the game as a whole.