We just finished our last 3.5 campaign (my bard laid the final blow on the aspect of Lloth, I'm so thrilled.) I've been playing 3e from the start, and D&D for 27 years.
4e is already, hands down, my favorite edition. Since the first time since shortly after D&D was published, it is on the cutting edge of RPG design. 3.5 always felt like the elaboration of prior editions. I appreciated the cleanup of the basic mechanics (AC vs. THAC0, DC ratings, etc.) but 3.5 ultimately suffered from issues that had always hounded D&D. I really feel as if those have been fixed.
ALL of my issues with 3.5 have been addressed.
A. Miniatures don't just look cool, they're fun to move around. Lots of miniature movement action. In 3.5 I used to joke that the minis were just for the first two rounds of combat, then everybody stayed very still moving only when opponents fell. Now the slide, push, pull, and otherwise jockey for position each turn. No more PCs just moving up and LAPDing some poor beast into the ground.
B. PCs are special. They operate on their own ruleset, which means I don't have to make the equivalent of a dozen PCs each week. I once spent three hours designing a villain that a single PC took down in three rounds, stunning him each round so he never got to do a single damn thing. Which brings me to. . .
C. DM friendly. Oh man oh man. It is so easy to make interesting encounters quickly. Skill challenges are a codification of what I've been doing for years. Finally I have a structure to players making investigations, crossing dangerous terrain, attempting to fortify a ruined castle, and all that jazz. Traps have logic behind them, and their use is dramatic instead of annoying.
D. Campaign structure. I love the Heroic, Paragon, Epic levels of play. High level play in 3.5 was so slow. Our group stopped the Ravenloft campaign because we couldn't do two encounters in one session. Play became that slow. I haven't done high level play in 4e yet, but what I see looks very promising. In particular, I like that ability "graduate" over levels so nothing is useless at high level.
E. Modular rules that are clear. I love that everything is so well defined and modular in nature. As someone who actively recruits people into the game, the learning curve is very daunting. The new game is very accessible, although there is still a lot, they learn it in increments by level. Turns a negative into a strength, which brings us to.
F. Better mechanics. Healing surges, at-will/encounter/daily powers, roles for classes. races have greater impact, action points (players always want more actions per turn), extended rests, the list goes on and on. Finally
D&D has a rule engine for, well, playing D&D. Stops silliness like characters buying CLW wands even though they can't use them, so they can give them to a guy who can use them, a backdoor healing surge if I ever saw one.
G. Dynamic combat. I love the groups of monsters in dynamic terrain. I was always pushing for this in 3.5, and it felt forced every time. Now I have a long range of tools (which will increase over time!) to produce interesting combats.
I could probably cover enough points to fill out the rest of the alphabet, but I love 4e and I'm not going back. I'm collecting all my 3.5 books (except the books I'm published in) and selling them on ebay for dinero to buy 4e books with.
Thank you Wizards for 4e.