For me the most important part of an RPG is the system. I tend to spend hours finding the best ways to play a certain system and find out exactly how it works. A bit vague I guess but using lets use one of the games on my wall. (looking for a game)
Resonance of fate: The system in this game revolved around three things (in no particular order).
1) How many bezel shards you had. (Determined special abilities in combat)
2) Your overall level (overall level is level in handguns + level in grenades + level in machineguns) which determined total weight you can carry.
3) Your gun customization which determined in battle attributes.
So most of my time with this game was spent figuring out the best way to customize the guns with the limited wight I had available.
Another game off my wall. (searching)
Tales of Vesperia: This is either the simplest or most complex game on the wall depending on how you play it.
I'll explain this game differently from RoF since it's far more complex. You can play this game two ways. The way where you fight you way through the game and do no extra arte or item searching eventually beating the game. Or you can delve into the games arte and crafting mechanics. If you want to get the most out of the arte and crafting mechanics then you have to learn about the enemies and the battle and drop mechanics. If want to get all of that then you have to do the sidequests for alternate versions and other such things. This game I would have to write a literal essay to fully explain the mechanics. Suffice to say in one sentence. The entire system of this game works together. Well. And I love it.