First, KFCrispy: I certainly agree that there are some stories that are better suited to games than to books, and while I also agree that description of battle is better done as a game than as a book frequently, RPG battle systems are frequently so far removed from real battle that when one tries to imagine it in real life, it falls flat completely. I would say that often a non-RPG representation of battle works much better as far as describing a battle goes. For example, people in battle simply don't take turns or jump forward, do an attack, and jump back. It just doesn't happen like that. Now, wandering around is better described by a game than by a book, if one is going to the trouble to describe every turn. Frequently, however, authors successfully describe wandering around by not describing it too much. Still, I'd agree that frequently a visual representation of moving around is better.
Another thing that I think that RPGs do better than books, as Chaco said, is having more characters. It's hard to have a book with large numbers of characters. It just doesn't work very well. Certainly having a picture to associate with the person makes them much more memorable.
I'm sticking with my previous statement, though, about RPG plot. RPGs tend to have more plot than other games (with some exceptions), but less than most books. While it's unfair to compare video games with Shakespeare, I do believe that it is quite fair to compare video games with, say, Lois McMaster Bujold, who manages to have very well-developed characters, engaging plots, and, on the whole, better stories than most RPGs. And she's just one of many writers of at least that caliber.