I agree a DT bookstore would have to be diverse. Definitely a nice art section, become a McSweeny's outpost; best sellers and a great newsrack to attract the masses; cards (fancy/nice- not CVS) and maybe some gifts (stationary, whatever); info for tourists/postcards/etc would be great.
I think there would be enough people to support the store now: office workers, downtown residents, NCSU downtown design studio, museum/F St. tourists, then think of the convention center, new hotels and condos. It would be a great addition to First Friday- say...15% off all art books!
If the store has something to really set itself apart (without losing mainstream appeal, of course), non-downtowners would come to it as well. As it is now, I go out of my way to get to Quail Ridge, and even Regulator, just to support cooler, independent stores. I like to think I'm not a freak.
I hope an indy gets in now before the big boxes really start eyeing downtown. (Though I'd kill for an H&M!).