Off the top of my head: a new set of BGMs (you can still use the old ones if you want, though), and the abilities to sell unneeded items and change the in-game character portraits to whatever you like. Also a certain character got a rather big boost to one of her attacks in between the last "official" version of the Plus Disk and the Special Disk, turning her from mediocre to possibly one of the best damage dealers in the game.
Actually, instead of possibly one of the best, she IS the best damage dealer in the game bar very rare occasions.
Another major perk is pushing the M key to guarantee a random encounter on the next step, a few bug fixes, a few new bugs (missing parts of animations for Miracle Fruit and Mountain Breaker, certain bosses having attack sound effects missing, etc).
Note for an RPG - it's HARD the first time through. Grinding isn't necessary except for a certain few bosses (mostly the last boss - you can beat the infamous 15F optional and 18F with the right team and strategy), as well as the Plus Disk. Also, each character has a niche, but you gotta play them towards their strengths in order to not have them be outclassed.
(e.g - Remilia is very fast, strong and durable, but so is another character you get later, and that character gets better options. Remilia only has two spell cards throughout the game, and one of them is a mediocre single-target attack. The other lets her stand out, by buffing up all of her stats by 50%, at the cost of paralysis and poison, which are preventable by gear or healable)
