"RPG? Like, MMORPG? Roleplaying game? Where are the graphics? Where is the game?"
Most games that hit the shelves and call themselves 'roleplaying' are not about roleplaying. No, really - take a look at the word: Roleplaying. These games aren't about playing a role as they are about playing a handful of quite generic options. That's not to say they don't have their charm - they do! - just that it is a slight misnomer.
With that out the window with:
Roleplaying in environments like IRC is about telling a story, especially when the method of roleplay is freeform. It's about people sitting together and playing truly miscellaneous roles in an effort to craft a story together.
Basically, it has more in common with being an author than it has with playing games.
It's still a bunch of fun - your fellow players will throw unexpected twists at you if you're not careful, and it's a sweet challenge to keep up sometimes. And at the very least? Good roleplay comes with the deep satisfaction of having crafted an awesome piece of literature.
So, what kind of mindset is it that helps enjoy roleplaying?
The most obvious part is not to consider it something you have to win at - at least in character. In fact, some of the most interesting characters in roleplay are created by the intense desire of the player to have them lose as often as possible - someone who enjoys seeing their character(s) fail, rather than succeed.
Beyond that, if you have any like of reading books, chances are (freeform) roleplaying is, indeed, your thing!