I really enjoyed the playtest. It got me much more excited about D&D again. The hardest part about playtesting was trying to evaluate new system/new mechanics and abilities without comparing to older versions. In one way, we all wanted to make sure it "felt like D&D," but in another way, when we kept on comparing it to other editions we had more difficulty being objective. Once my group and I could focus on just this edition (and the options that WotC presented) without comparing to old, we started to really enjoy the way the game worked. Of course, as each new package was made available, we had to shift gears and relearn and re-feel what was presented, but that too became interesting.
Another issue that bothered many was how they didn't balance monster math during most of the playtest. At first, it was hard to get used to, but when I realized that they consciously made the decision to test "feel" rather than calibrate math throughout the playtest, I was able to play with monsters and basically test different ideas on my own. I found that it was so easy to modify and change encounters on the fly, which was a portent of the "rulings not rules", flexible play that 5e affords. Once I DMd about 3 or 4 sessions, I felt very comfortable with adjusting numbers if necessary. That's not to say I did it all the time. I also played with RAW to make sure I was able to report on the results.
Many of the earlier mechanics are part of the optional rules in the DMG. I really liked how proficiency bonus was an extra d4, d6, d8, d10, etc. as PCs gained levels, but since I use Fantasy Grounds for most of my games, it is harder for me to run that option. The rules in FG are already hard coded for fixed proficiency bonuses.
I also found that the adventures they game us to play with worked really well. Caves of Chaos was a great sandboxy backdrop that allowed different groups to do with it what they wanted. My group actually made an alliance with one of two orc tribes and helped them solve a conflict that stemmed from one leader trying to take over both tribes. This just happened organically in our game. The adventure did not have this prompted.
The Blingdenstone adventure was also a fun mini-campaign that got us into the Underdark. It was really easy to add my own material and use what WotC offered. Again, this to me is what D&D is best at - allowing groups to make it what they want and exercise creativity as players and DMs.