I remember sitting down to Chrono Cross for a 15 hour session and thoroughly enjoying every second of it. I remember the feeling when you discovered Tidus was just a dream. I remember the joys of knowing that I'd cleared the FoW and mapped 100% of an area on Star Ocean. Suddenly, I find myself unable to play a RPG for any great length of time. Even a couple of hours begins to grate on me.
Shooting games however? I can dive neck deep in Vanquish. I can spend hours conquering the Covenant. I can spend whole days running and rerunning Army of Two and Borderlands. So, what happened? Am I a product of the times? As stimulus has increased and moved faster and faster, I need my entertainment to do the same to compensate? Or, has my brain somehow malfunctioned and I now require a lesser amount of game to make me satisfied?
Maybe Mass Effect 2 will help. It's a huge RPG... but you also spend a great deal of time shooting your way through things.