Good:
Speed is better
Awesome music (Bramble Blast, Dedede's Theme)
Melee clones are more "Luigified"
Buffed lower characters (Kirby, Bowser, Luigi, Zelda, Game & Watch, DK, etc.)
Nerfed better characters (Fox, Falco, Sheik, Peach, etc.)
Assist Trophies are a cool addition
Final Smashes are a great original concept, and help balance some characters
New techniques (crawling, gliding, footstool jumping)
GCN controllers
added back features from 64 (entrances)
actual random button
3 taunts
Online Play
Bigger roster
David Hayter as guest VA
Larger Adventure Mode
Bad:
Online play is BALLS
random tripping
focus on casual audience
the axing of an original moveset (Mewtwo)
lack of new series (Golden Sun and Nazo no Murasamejo could've worked great)
12 to 26 characters is much better than 26-39, despite clones
Lack of Ridley, Bowser Jr., K.Rool, Krystal, and Isaac
Overrepresentation of Kirby, Pokemon, and Star Fox
Underrepresentation of Mario and DK
No new DKC stages, but representation of classic games and crappy bongo spinoffs
Biased on Kirby series (menus look similar, subspace is focused on them, 5 items, more trophies than anyone but Pokemon, etc.)
Nerfing of characters who didn't need to be nerfed (Falcon, Ganondorf, Jigglypuff, etc.)
Overpowering Snake and Meta Knight (Kojima and Sakurai's mascots)
Cloned Final Smashes
No Ryan Drummond as Sonic's VA
repetitive adventure mode, with no Nintendo worlds, and only Mario enemies
Chaingrabbing
Few original bosses
Less ATs
Sakurai focusing on distilling gameplay instead of adding features