There are a few things about Brawl I don't like that much. First up, I'll talk about stages:
New Pork City: This area's just too big, in my opinion. I'll admit, the size can lead to strategic gaming, but I don't think that's what the Smash series is supposed to be. The Ultimate Chimera also causes problems.
Mario Bros. I HATE this stage! First off, it's automatically fixed, so you have to keep a close eye on your character. This can be overlooked, however. The crabs and turtles, cannot. They're highly overpowered and can kill you very easily. Due to this reason, I usually stay clear of this area.
Rainbow Cruise/Ride: I don't like this one for the same reasons I didn't like it in Melee: too much Forced Scrolling. I never liked stages where you had to be mobile to keep up with the stage instead of getting away with the enemy.
Rumble Falls: Just like Melee's Icicle Mountain, Brawl's Rumble Falls is an auto-scroller. The only good thing I can say about this stage is that it only goes up. Other than that, though, it's worse in the fact that it's got hazards like spikes and other stuff on the stage.
Spear Pillar: Don't get me wrong, I like this stage, but Palkia makes things kinda confusing when it flips the stage around. It's kinda like the Psycho Control Curse in certain games, except it's mostly mental this time around. Also, Cresselia's blades are a bit powerful if you don't dodge them.
Last, I'll note the items that I don't like:
Cracker Launcher: Just a stupid broken item. It's powerful, yet if you get hit even ONCE, you immediately drop it. Whenever I see this, I just toss it out of the arena.
Deku Nut: Another cheap item. Admittedly it's got a drawback, but that still doesn't excuse the fact that it's powerful when in the air.
Dragoon: I'll say it just once: Dragoon = CHEEZ! It's only drawback is that it's got three parts to it. Other than that, it's like a super-fast Home-Run Bat! That's why it's my most hated item in the game!
Fan: I don't hate the item itself. I just hate how people spam the normal attack with it. In a way, it's really cheap.
Lightning: I hated this item in Mario Kart, and I hate it in Brawl. In a way, when used correctly, it's like a Poison Mushroom for everyone that lasts longer. There's also two Backfire Modes for this item: the Regular Backfire and the Power Backfire. The Regular Backfire shrinks the user instead of everyone else, and Power Backfire is the opposite of the normal mode. It's a cheap item that's unpredictable.
Smash Ball: Sure it adds a highly powerful attack, but come on, people! Some attacks are kinda overpowered compared to others! Especially Marth's Critical Hit. If I EVER catch you using that, I'll boot out immediately! I LOATHE Insta-Kill attacks!
Soccer Ball: An insanely powerful item that only needs to be smashed for it to work. Usually it takes one fully charged hit for the enemy to vanish, so it's kinda cheap...
Timer: Almost the same deal with the Lightning, except it's Chaos Control instead of Poison Mushroom. The Backfire Modes also apply here, except Power Backfire slows EVERYONE down, like if they were under the influence of Dialga's Roar of Time.
Urnia: At high percentages, this item can and WILL kill if used correctly. Also might have the potential of a Meteor Combo, but I'm not sure about that.