I like a good story with plenty of choices that have actual impact. It doesn't all have to factor into the ending, but the choices and their consequences are what make multiple playthroughs so appealing. I can forgive some gameplay flaws if the storytelling is good. Alpha Protocol for example is a pretty flawed game gameplay-wise (and those bugs, oh god those bugs..), but I just love the hell out of it despite that. Plenty of choices, and those have actual impact. Even the choices you don't know you're making can have impact, at least on a conversational level, and because of that it's always fun to start a new playthrough. That is something I wish so bad that other game developers started doing. In, for example, Mass Effect 3, your personality makes no difference to anything, and all the choices that make that playthrough different are made in one line of dialogue that you pick. I still love the game, but Bioware could still learn something from Alpha Protocol. And I certainly hope PE will make use of those lessons too! I assume many of the team worked on AP after all.
But good gameplay certainly makes any RPG that much better. Torment for example is just a step away from perfection because the combat is so tedious.