1) A new edition is a significant investment in money, time, and thought. I've already invested quite abit into the 3rd edition. To move from it, I need an actual compelling reason. I haven't got that so far. I'm simply not going to move just because its the latest new thing. I'm rather surprised at how many people said that they would move to the new edition sight unseen.
2) The new edition seems to heavily emphasize D&D as a board game or tactical wargame. Quite a few people on the board have praised that, but if I wanted to play a skirmish game or a board game I would. I want more depth from the rules set, more 'simulation' if you will, when I'm playing a RPG. If I think they are getting in the way of the players imagination, I'll remove the minatures from the board. With the heavy emphasis on shoving the enemy around the board to line them up in your area of effect, it seems like it will be even harder to abstractly run 4e without minatures than it was in 3e. All I really had to worry about in 3e was conceptual adjecency (to run AoOs).
3) The new edition seems to be going for streamlining the rules set. Rather than fixing problem areas in the rules, the general design paradigm seems to be 'if it causes complexities' do away with it. I want a complex game. There is always a loss in resolution by going simplier. Third edition seemed to me to be a very happy balance between ease of play and preparation, and simulation depth. I know alot of people wanted a 'simplier' game. I wasn't one of them.
4) The new edition is wedding way to much fluff to thier crunch. It's redefining alot of the games historic elements. This is perfectly fine if you are a new player or you just play WotC games 'out of the box', but some of us have been playing for more than 25 years now and we have existing ideas about flavor, setting, and other conventions. Adopting 3e invalidated almost all of my 1e house rules, but it didn't invalidate my flavor. That's not clearly true about 4e.
5) To be honest, Mearls is not my favorite designer in the WotC stable. He's got interesting ideas, but I don't like alot of where he takes them. Specifically, I see 'Mearlsian' design as featuring lots of tokens to keep track of (only Mearls would think 'petrification tokens' are an elegant solution to 'flesh to stone), a balance between spellcasters and non-spellcasters by making all classes spellcasters in everything but name, and having a sweet spot geared toward the introductory part of the game. His stuff looks like it plays great at 1st level - probably better than D&D has ever played at that level. But I'm not so sure that its actually simplier or better than what we already have at higher levels.
6) The game seems to be tamer and safer than what I'm used to playing. Alot of the things that made success difficult are being removed from the game. This is highly conjectural on my part, but I have a strong feeling that the new edition is relying heavily on 'tactical illusionism'. Normally 'illusionism' refers to a DM technique where the players are made to feel that the have free will because they are being presented with a great many choices. However, the DM is secretly dictating that every choice actually leads to the same outcome. For example, the PC's encounter a fork in the road. The two roads look very different and head in very different directions. But not matter which one they take, that road will lead to the 'Lost City of Foorgidor' and the DM will adjust his map accordingly. By 'tactical illusionism', I mean presenting players with a reasonable number of seemingly relevant choices in combat, however none of these choices is actually both critical and difficult to discern. The player feels like they are making crucial choices, because they achieve a great deal of success, but in fact there isn't much difference in outcome between highly skilled players and novices. I can think of several games that are like this, for example, Cosmic Encounters, Bohnanza, and several variaties of dominoes. The game superficially appears deep and is emmensely fun at first, but after playing it a while you realize that for various reasons it really isn't that interesting. I think 'per encounter powers', the removal of effects that tend to steal 'turns' from participants, the standardization of what you can do in a turn (it seems everyone has a move and a attack), and so forth lends itself to this sort of illusionism. Once you realize that in every fight you are basically doing the exact same thing (a feature probably hidden by the rumored rapid advancement), I think its going to wear.
7) Some of thier specific choices seem really wierd. I can somewhat understand doing away with Vancian magic, but replacing it with a highly inflexible and limiting 'per encounter' system when much more flexible options had been devised for 3rd by various vendors seems bizarre. The notion that clerics 'need' to attack every round, and so hense thier successful attacks heal thier allies just irritates me as someone who enjoyed playing clerics in 3rd. Apparantly 1st level PC's will have multiple HD, but we don't seem to expect 1st level NPC's to have multiple hit die. This is just a wierd way of stating that starting PC's are actually (in essense) third level (especially because we can assume that they have other advantages over NPCs). I pretty much know why rings can't be used by characters below 10th (we'll have to get used to refering to 9th as 'low level'), but even if I understand the reasoning taken altogether it just seems wierd and out of place.
8) I don't like the 'take it up to 11' they've been doing with fiends. It appears that 'Orcus' was in fact a quite suitable codename for the project. I know alot of people think this is much of what makes 4e cool, but it just seems lame to me.
9) Probably lots of stuff I forgot. (Like for example, reading through some other people's post I'm reminded how much I HATE the art previews I've seen, and how much they seem to tell me about the age of 4e's target audience.)