Dude, are you kidding me.
+Costumes: Being able to personalize each of the characters in battle is awesome, and the in dungeon conversations over them are really cute. It only adds to the replay value. It's pretty much the reason why I use Teddie in battle (Bishie human Teddie > regular Teddie)
+Two new social links. Marie (who's blargh overall, but her social link events are occasionally good when the other characters show up) and Adachi's (who's social links are great and add further depth to his characters)
+New personas and a scene beyond lvl 10S.link for the investigation team members. Sure, you might not find use for these abilities, and you get to use these abilities only in 2 dungeons, but these new skills end up making each of the characters "more fun" in the long run.
+Scene fast forward(why hello there, new game+)
+A dozen new Personas like Kaguya.
+/-Some new events that are hit or miss. Skiing trip was a fairly good event imo the game.
+The TV listing feature which adds a TON of new content (Music, concept art, quiz game, a minigame, etc)
+Rebalanced characters. Naoto is usable now(she was really awful in vanilla) and Chie actually has a couple of reasons to be used over Kanji now, and the characters who lost their weaknesses like Yosuke got them reversed (why on earth did Yosuke lose his electric weakness when Yukiko kept hers?!?)
+The bike plot that initially went nowhere is now a thing. Everything associated with bikes (new skills, okina city and the beach) are great additions.
+Night time events. You can use them for boosting S.links, but simply talking to your friends at night is a reward in itself.
+It's possible to hang out with your friends after you've maxed out social link, which shuts down the "max out S.link and never talk to them again" mentality.
+Several unmentioned tweaks to the game. According to buddha, there are a couple of these scattered throughout the game. The biggest one that comes to mind are the scenes that are changed if you have a girlfriend. (It's only a line or a blush, but it helps drive the point home that "X" is your girlfriend)
+Golden ending.
+P4G has more of "hanging out and being silly with your friends" which is my favorite aspect of Persona 4, hands down. So much fun.
+/-Difficulty
The game is easier overall, but you know have access to "very hard" difficulty, which is somewhat challenging if you don't grind.
As an aside, I've always seen Persona 4 as an "easy game" once you get going. It's difficulty was accentuated by some of its more archaic game design choices, but overall P4 is an easy game once you understand the mechanics. People harp on Rise for breaking the game, but it's pretty much impossible to lose once you get some of the ultimate Persona (which is around Rise becomes ridiculous). The changes to some of the bosss (looking at you Yukiko) make easier for new players to get their groove on.
P4G is the whole package. That golden RPG that's supposed to last you 200+ hours.