Best:
- Exploration, including lots of hidden rooms, nooks and crannies, not to mention lots of treasure chests and other little things (like the flowers and rabbit hole treasures in Wonderland, the candles in TT and the fires outside OC, the treasures on the Clocktower, trinities and dalmatians, etc.)
- Difficulty. Even the minor Heartless pose a challenge. The Disney bosses aren't complete jokes next to the original bosses.
- The storyline is incorporated throughout the worlds in minor or major ways. Finding gummi's at Wonderland and Deep Jungle, Riku's involvement with Monstro and Neverland, learning about the Keyblade at Atlantica, etc.
- Disney and FF characters have integral parts in the story.
- The music, graphics, and voice acting.
- The tournaments at Olympus Coliseum, particularly Hades Cup.
- Magic system/spells.
- Dodge Roll, MP Gift, and tech points.
Worst:
- The gummi ship absolutely sucks (and still does, as of further games, imo).
- The mini-games are few and they're mostly clunky. Although I'm not really a fan of mini-games anyway...
- Fish face.
- A lot of characters without voice actors, although that's bearable.
- Summons that are useless outside of Tinker Bell.
- Alice being a Princess of Heart instead of Ariel.
- Party members not knowing how to use items, magic, and abilities coherently.
- Jafar's boss battles. He had the most potential of the Disney villains, actually, and yet his fights are the weakest/least interesting.