Ah, time for another long winded post.
1) I dislike the fluff STRONGLY, but FEAR: I'm worried the fluff will be somewhat forced on us. In 3.x, one of the (odder) points of contention was the lack of fluff, which I really saw as a GREAT thing. It meant you could near seamlessly shift things around. But if the mechanics and fluff are too intertwined, like I'm starting to feel in 4e, it makes shifting things a lot more difficult, and I loves me some homebrew.
2) I've always enjoyed fluidity of combat; not just in actual mechanics, but in style, as well; yet with the large emphasis on the battle grid, FEAR: I'm worried that fights will lose their adaptiveness. One of my fonder memories in a game was having the party fly around a truly enormous monster in their air-ship-thing, pelting it with spells and arrows, while the paladin (who had been playing too many video games) jumped on it from above and began climbing up, a maneuver that culminated in him using it's hair to swing down and kick out it's eye. All this was done almost completely on the fly, all of it was done without grids, and it worked FANTASTICALLY. I'm worried 4e will greately reduce the ability for players to work and think outside the box. I have no problem with grids - I use them from time to time myself - but I do perfer the ability to CHOOSE between using and not using grids.
3) Skills. Skills skills skills. If I loves me some homebrew, then I REALLY loves me some skills. As I mentioned before, I strongly encourage players to work and think outside the box, and to be very versatile in how they do things. Now, I'll be the first to agree that 3.x had way too many skills, some of them completely useless, others of them that should've been combined together. But FEAR: I'm worried that the skill list has been hacked down way too far. It was a common joke in my groups that one day skills in both tabletop and video games would be dumbed down to "FIGHTAN, STEALAN, and MAGICKAN." Unfortuantely, I'm starting to worry that 4e didn't quite take that as a joke. To use the aforementioned example of the paladin who had to have been listening to the Shadow of the Colossus soundtrack in his head, he had planned in advance for this by rummaging through the group's Giant Mound of Magical Items and replacing some of his current equipment with things that increased his balance, jump, climb, and tumble. Part of the excitement of the fight was seeing him just barely manage to roll over each DC as I described his ascent, along with the occasional curve ball I'd throw at him because, well, we can't have our players getting TOO cocksure. If the skills are reduced so drastically, I'm afraid we'll lose these chances to really let the more creative players shine.
4) Classes. If you're seeing a common complaint, then you're reading what I'm trying to convey. One of 3.x's flaws was also it's biggest selling point - versatility in leveling and classes. Yes, it led to a *very* large power creep issue, but even then, players could and would make the classes that really did best define their character. That's why FEAR: I'm worried 4e will greatly hinder the process of making the character concept YOU want. I was extremely dismayed to see how multiclassing was being handled, and I continue to be dismayed at how players seem not to gain new powers, but merely replace old ones with "better, new powerful versions." The name of the game with tabletop gaming should be creativity. If I wanted to play Hackmaster, I'd play Hackmaster. And on the note of feeling dismay, what is this nonesense about no bards? And the, in my opinion, asinine move to take monk and barbarian, classes that have been core for a DECADE, and make you buy a seperate book for them? Lastly, classes are looking too boxed in. One of my favorite classes was the swashbuckler, in no small part because I'm a total fan of pulp. But it seems the swashbuckler no longer exists. While I can see the strengths in wanting to make classes stand out in terms of archtypes, it's far too thin of a line between archtypes and stereotypes, and when you homogenize a class, it gets very boring, very fast.
5) Magic items. I mentioned the party's Big Mound of Magic Items before, and that's a staple in many of my games; the idea of "Don't sell ANYTHING, because we'll totally use it eventually." Honestly, who would have thought the belt of increased balance would come in handy? Well, the paladin did, ,that's who. Magic items in 3.x had the same problems as classes - too much power creep, too many things working together that weren't intended to work together. But having that huge variaty was a good thing; more choices gave more options for more creativity. Now, I haven't read much on what items 4e will have, but FEAR: I'm worried we'll definitely see much less, and those that remain will be much more homogenized.
6) Did I mention I dislike the fluff? I'm totally mentioning that again. Tieflings, dragonborn, and teleports, oh my! The goal is to make things seem cool and sexy, but it's starting to sound almost like a joke. We're gamers, we couldn't be cool if we had a +10 bag of social conformity. And making your game into a picture of a guy in sunglasses jumping out of an airplane while chugging Mountain Dew doesn't fix that. Thus, FEAR: I'm worried that 4e will try to make things too "fantastically awesome!" The problem is, when everyone stands out, nobody stands out. When everything is over the top, it gets bland. Again, if I wanted to play Exalted - which I don't - I'd play Exalted. How is the human player going to feel when he says "I add more damage using my action points!" followed by the dragonborn player saying "I use my own action points to BREATH FIRE!" Or the dwarf that says "I stand my ground and push the enemy back!" right before the Eladrin says "I magically teleport behind the enemy!" How are they going to feel when this happens at level 1?
Edit: I should add: I don't dislike 4e, and I'm definetly not trying to be one of the stock haters that wander around, waving their cane at the youngun's these days with their cell phones and their rock and roll. I'm on the fence. Well, I'm on the fence over mechanics. I'm very far away from the fence as far as the fluff is concerned. But hey, that's what homebrew is for.
Further edit: my spelling really is horrendous. Also, typos EVERYWHERE!