Finding every little thing in the game and trying to complete every quest, side story, along with getting all the achievements, which ends up with me not having fun anymore. I end up spoiling the story going through faqs, because I don't want to miss anything. Damn me and my completionist self. My favorite genre has become a chore. thanks achievements!

I would probably have to say it's the gameplay that first and foremost defines an RPG so it is probably the most important aspect of an RPG ...

Story can be a positive but it can ultimately stray the supposed "RPG" into some "visual novel" ...

Agreed. I have never powered through an RPG for story if the battle system is awfully. I have powered through a story is the battle system is great. Case in point is Grandia Xtreme, the weakest title in the series, but the battle system is still sublime.

Battle system. If it sucks or is too confusing, I won't play it.

Story/characters (not story/lore) and combat system are my 1st and 2nd options. I'll stomach a bad game with a good story, cast, or combat system.

That's good, but it's the opposite for me, the generic gameplay that Suikoden 2, FFVIII, FFIX never bothered me nor liked it but the main and side stories was what pulled me to playing RPG's during the day. RPG's now I find it to be too focused on flashy moves and gameplay(Tales Series/ FFXIII-2 became QTE gameplay) that after I got to the point of the game where it gets really boring with no story to back my motivation in continuing the game. Well that's how I feel about it atleast.

Good battle system and action pack with good storyline. >.>

Hmmm. Interesting, I read through the thread before posting. I didn't see any mention of the two things i find the most important in a rpg. Techinally any video game is capable of either of these but in rpg's it's the eastiest and most common to find.

For me, the two most important aspects of rpg's are immersion and gamer euphoria. Immersion can be achieved by any combination of aspects of a rpg. That combination can be different for each person but the end result is always the same, the better immersed someone is, the more they'll enjoy the game. Any style of rpg can do that. Sometimes it's the story that pushes you into complete immersion then other times it might be a particular battle that's fun. Sometimes just fishing in a game will do it. Glitches and bugs are the bane to immersion though. 

Gamer euphoria is harder is achieve and it's fleeting. First, it can only be anchieved by doing something challenging to you within a game or by some goal that you set out to accomplish and doing it. You can't keep repeating the same thing and get it again. It requires a fresh goal or experience. Some gamers know what it is and most don't. It's probably the best natural high out there. haha. The easiest way to experience it is by playing difficult rpg's. 

Customization/upgrades, World shaping (influence through actions) and development of characters and groups through choices or conversations. The battle system can differ as it seems to be the case with modern RPG's. The idea of what an RPG is..is quickly evolving.

everything!, but if i was forced to chose one then the combat! DeS and the Tales series are some of my favorite JRPGS/RPGS because of how fun the combat is, and how fast it goes.