I've always respected and admired the technical achievements and fun of the GTA series, but the theme always kinda bothered me. I never really could get into being a gangster or crime lord. It's one of the reasons I enjoyed Crackdown more than GTA IV, despite GTA IV being a deeper more beautifully crafted game.
Red Dead Redemption takes the great technology and open game-play concepts of GTA and places them in a setting that is much more appealing. While I'm sure it will be possible to be a 'bad guy' in the old west, it brings to life a genre that has been ill served to date by video games. To finally have a western themed game that has all the polish, attention, and budget of a GTA title is very exciting for me, and probably one of the reasons the game was so highly anticipated by many.