Liked:
Character development of characters who needed it, meaning just about every non CoM member.
Gameplay
Multiplayer Mode
Story
Diary/Secret Reports
Panal System
Limit Break System (Loved it more than KH2's more spammier, abusive system)
The comedy this game brings. This KH might be the (only?) KH that brought me the most laughter.
Et cetera.
Disliked:
Lack of boss fight design. KH2, while had a mediocre story at best, had excellent gameplay and boss fights. However, most bosses in 358/2 Days are pretty repetitive. Leechgrave was about the only difficult boss, while there were about two other fun ones (or, almost any boss that wasn't a palette swipe was decent). I was vastly disappointed with the game's final boss as he was extremely easy. I was hoping we'd get to fight a second form, while being much harder and winnable, would lose in a cutscene, but that did not happen. At least, the bosses in this game were generally no Kurt Ziza or CoR bosses. It is the DS, so I can give it a break, but even CoM had Ansem and Riku IV battles that had a wide range of attacks.
Most of the enemies are simple palette swipes.
Some enemies will disappear when they travel too far out of their area, and respawn at full health. This is especially annoying for some of the aerial enemies who seem to run from you.
Generally, most of the new keyblades in this game did not look too pretty. This isn't much of a problem thanks to Zero Gear, which is the only gear I use.
I recently realized, though can say so officially, but the replayability of this game might be less than what I thought. This game is nothing but missions. That in itself isn't bad, but that's pretty much all you do when you do the extras. First you do each mission normally, then you do its holo mission to get previously unobtainable badges, chests, or to complete it 100%, then you do its challenges, which are sometimes two for one mission, then you have it in multiplayer mode. That's really not all that bad by itself, but the purpose of starting a new game, while it does have its pluses in wanting to re-experience the story, doesn't have much else. In most other KH games, being a person who loves the gameplay, I'd love looking forward to fighting a certain boss, but that's countered by it can easily be done by selecting its mission in another data, and the game mainly only has bland heartless bosses to offer, for the most part. I'm currently playing the game again on proud, and that's what I'm feeling with the missions since for the most part, I've done these many times trying to get more sigils or in multiplayer mode.
All in all, the game's pluses greatly outweigh the negatives. Most of the negatives are because its on the DS, I assume, and this game is superb compared to most other DS games.