Birth by Sleep was a revelation for me in my Kingdom Hearts fandom. The first game where you didn't play as Sora, AND taking place before (at the time) any other game in the series, having to play the same main story arc from three different perspectives was interesting. Also, getting to see the beginning, middle, and end of a particular world's story from said perspectives was really neat as well, though when you take a look at the timeline on the main menu, the timing in visiting these worlds seem out of order. But I digress...
The command deck was an interesting way to get around the shortcut menu and combat system on a portable device and while I don't have a problem with it, I do prefer the series with a legitimate controller. Getting to see certain Disney classics was interesting, though I prefer some of the more modern selections from the mainline games.
Seeing as how THIS game would turn out to be the secret movie from Kingdom Hearts II was surprising for it to be a PSP game, but it's probably the most important Kingdom Hearts game in the whole series that is NOT the main trilogy. One final thing I actually like is the monopoly-like mini-game where you can powerup your commands and earn items so you can fuse them for even more powerful attacks. In terms of difficulty, the order recommended to play is start with Terra, move on to Ventus, then finish with Aqua, so when you go to the first world as Terra, there's a chest that's difficult to get to, but if you get it just right, you can ultimately obtain MegaFlare and be OP for the rest of his campaign.
There are some things I don't like, for example the Mirage Arena (which is this game's version of the Olympus Coliseum), Disney Town, D-Links, and a couple of story beats, but overall, Birth by Sleep is always a fun time to play. Sadly, because I'm terrible at games, I've never been good enough to play the Secret episode, only watch it on YouTube, but all in all, there's so much here that's really good. My second favorite PSP game behind Crisis Core, and one of the best Kingdom Hearts games overall.
Shoutouts to @FF7Cloud, @Jamicov, and @Sheria for enjoying Birth by Sleep to the point of giving it votes!