Playtesting is good, and I'm sure we'll all be doing some amount of it over the next year. That's awesome, and I really can't wait. In the meantime, we should also be looking at diverse RPGs and really getting a handle on what we like and dislike in a roleplaying game. It's not enough to know what we've liked in DnD, we've got to get a handle on what things contribute most to making a solid RPG. Especially if the goal of dndnext is to make something that supports the desires of everyone from the old "DnD as a Chainmail Hack" days all the way to 4e.
That's one helluva tall order, but I think we can do it. So, here are some recomendations that I think will help us all get a better handle on what we're looking at, and what we could do.
Old School Hack has some very innovative takes on core concepts, but very much plays like a fun session of the old Red Box DnD.
About as far away from 4e as you can get without play something that isn't DnD, really. It's basically Red Box with the kinks worked out, and an extra dash of awesome. :P
The One Ring, even if you aren't a huge Lord of The Rings or Hobbit fan, this game is fantastic. It's low magic, as you'd expect from a LoTR game, is very narrative driven, but has very solid crunch as well. Your socially savvy explorer is going to be just as valuable as your mate's brutish Beorning bear-skinned berserker, or the wise scholarly Elf of Mirkwood. The combat mechanics as simple and easy to use, and the out of combat stuff is very sophisticated and well thought out.
The Dresden Files RPG is one I've not played yet, but that won at least one ennie last year, and is highly recomended. I plan on playing it a bit early this year, if I can get a group together.
Pathfinder really is 3.5, but better. There is some obvious cross polination of ideas between PF and 4e DnD, but none of the 4e ideas that seem to show up in PF should coincide with most peoples' problems with 4e. The game feels about the same as 3.5 does, but with a noticably wider "sweet spot" and a reduced range of imbalance. I find it clunky, just like I always found 3.x clunky, but if you enjoyed 3.5, PF should be a similar, but slightly better experience(Not just me saying that, it's everyone I've ever met or talked to who loved 3.5 and has played PF).
ANd of course, PF is all OGL, so you can check out all the rules for free online before deciding if you want to play it. Finding a store with a game running usually isn't too hard, depending on where you live.
Free rules online: www.d20pfsrd.com/
Help the next iteration of DnD by knowing your game, and what specifically you like and dislike! The more we all know about gaming and our preferences, the more we can help WoTC make dndnext the best damn DnD ever!