Disclaimer: I am not a beta-tester, just someone who went to DC and played some 4th Ed this weekend. I figure only a small subset of you will care.
So, how much did I actually see? I got a pretty good feel for how the game will work at low-levels. There were 6 pregenerated characters, so we saw all but the Rogue (which was spotlighted on the website anyway) and the Warlord (which will do the same sort of thing in combat as the cleric, but differently).
First impression: I like it. First level was actually fun to play, and I felt like I was my playing class from the get go. The new power system means everyone has options, and pretty much every class always had at least two different at-will powers to choose from. Did I mention this was first level? The ranger was dealing death from afar, the wizard was throwing area spells, the cleric was healing while making potshot prayers for second rank, and the fighter was pretty good at keeping the monsters from the squishy characters.
Instead of limiting healing based on the clerics, each character has a certain number of times per day they can get healed, with each chunk being 1/4 hp. PCs can use one such "healing surge" each combat as a standard action, and can use as many as they need out of combat (during a 5 min rest). Clerics can give additional combat healing, but it still comes out of the daily surge total. The death and dying rules do make each turn you're unconscious exciting, while at high levels the death threshold of -1/2 hp means you're more likely to die from the new bleeding-out system then just drop from fine to dead in one hit.
The designers have also discovered that putting up a few abstraction barriers would make game balance and their lives much easier. Everything a player needs will be in the PHB, including magic items. If there is a polymorph-like effect, the stats will be in the PHB. If summoning is included (it won't be right away), the stats for those creatures will also be in the PHB. Monsters don't get feats, just abilities unique to the monster in question. (There will be rules in the MM on how to turn some of the monsters there into PC races. This includes Gnomes).
Finally, they didn't want to say much about multiclassing. It seems like you'll have one primary class, and can splash in flavor of other classes by taking feats, but there probably won't be a way to be a fighter 2 / rogue 2, for example. This means that skill will be less strategy (combing your splatbooks for the best feats/PrCs to make a powerful build) and more tactics (figuring out a party dynamic for combats, and adapting that as the nature of the threat becomes apparent). While I enjoyed the strategy side a lot, the time I spent thinking about how to optimize my character build was somewhat excessive compared to the amount of time i'd actually be at the table playing said character.
But yeah, if you have specific questions, feel free to ask. I can talk about anything I know about, save one fairly inconsequential question I asked a WotC designer that I promised to not take to the Internet.