To me it doesn't matter if the gameplay is bad all that matter is story and character. This always happens to me on tales games i cant never finish them because of the bland story and characters.

GAMEPLAY!!!!!!!!!!!

If your going to need to grind the same dungeon, at least make the gameplay fun.

Battle system and characters make RPGs to me.

Seamless adaptation of the upgradeable abilities/powers to the character.

And a good story without elaborations.

Pacing, exploration and feeling of progression. I rather have a couple of hard fights instead of tons of grind filler. The pacing should be so you don't need to grind.

Getting lost and learning the layout of the land or dungeon is more fun than following a marker on the map. The best times I had was getting lost somewhere in a side dungeon in FF12, barely managing to keep up with the respawn rate trying to get past a mini boss to find the exit. Same in Dark souls. Not knowing where I ended up and slowly getting to know my way around (after numerous death) to find my way onwards.

What kills an rpg for me is endless sidequests aka todo lists, Kingdom of Amalur ugh. And forgettable dungeons, for example Skyrim. The overworld in Skyrim was great, dungeons sucked.

-Lots of side quests.

-More optional overpowered bosses that will give ultimate weapons/items/armor if you beat them.

-Various playable characters and their customization. (the more the merrier)

-Good story writing.

It's hard to say because RPG rely on more than one aspect to be good, there not like, say beat em ups or FPS which are very simple compared and only really need good gameplay.

RPGS on the other hand take up a lot of time and effort so they have to have in depth characters! (Your role playing at the end of the day) also the story has to be gripping and deep, these two IMO are the most important factors. But it's alright liking the characters and wanting to know what's going to happen next in the story! You still need to be entertained with the gameplay and battle system in between, and as you rightly say, the music has to be correct and beagle to give you that feeling of nostalgia when you go back to it years later.

It's actually a tie between story and gameplay! I'd say that both are almost equally important in a RPG!

it should have a good mix like last remnant

Weapon & Armour progression with satisfying combat.