Samus had some poor transitioning, and there's also the fact she's made a little too comedic for my taste (I'm mainly talking about her incredibly stupid sprint animation). All of the originals have changed so much. Notably, Luigi is less of a Mario clone and actually kind of a powerhouse when in the hands of a skilled player. However, his recovery is harder to pull off, and it's common for a non-human player to screw it up, something you'll see happen frequently in Melee specifically. Mario's down-B got replaced with the Sunshine hose (a fairly useless attack) and then his aeriel down-A is now what down-B used to be (which sucks since both of those attacks were powerful). Then Captain Falcon's Falcon Punch has been slowed down (it's not by much, but enough to be noticeable), even though it was balanced already with the damage dealt even enough with the time spent charging it. Probably one of the good transitions was Link's recovery, which wasn't worth **** in the first installemt of the series.
And I'm not sure if it's just me, but the computers are less agressive. They are challenging and tough, but lack the in-your-face tactics from Melee. In SSBM, if you put up your shield, the opposing CPU would go for a grab. Now they don't necessarily do that and often leave themselves open to attacks.
Of course, my least favorite aspect of it all is between Wi-Fi or Stage Builder. Not only is the piece limit ridiculous and unneeded, but the computer players generally play poorly on any stage that's made to have any cool depth or design. If you've played Brawl's Wi-Fi, it deserves no explanation. Very rarely do you find players that aren't teabagging friendlies that probably couldn't grab a Lv.1 Ganon if they tried.
In light of this, I haven't played Brawl in quite some time. Or my Wii console in general.