I've always loved GTA, but I've only ever actually managed to finish one game (GTA4), and that took me a year and a half. Read Dead Redemption must clearly have done something right, then, to get me to attain 100% completion within a month.
I guess the main thing that made this so fun to me was the setting. To me, the Old West is a much better fit for the Rockstar open world design philosophy, though I can't quite explain why. It just felt perfect, exactly like the ideal western game should feel, with side quests in the right places with the right mechanics and a main quest that spanned across exactly the right environments, themes and story elements. So on a more abstract level, this is a really addicting and atmospheric game.
But there's also the more concrete stuff that made this so fun. Ambient challenges, random world events, jobs and main story missions combine perfectly to cover pretty much everything you'd want to do in this type of game, while also giving you the feeling that the world you're roaming around in is a real one. The story itself, which is much more serious than Grand Theft Auto while still retaining a hint of humour and irony, is also much more interesting and deeper than any of Rockstar's previous games. And of course, the world design is fantastic, with truly memorable scenic views, amazing animations and wonderful graphics.
Really though, I think what made this so much more fun than GTA4 to me was the freshness of a new setting and style. So bring on L.A. Noire!
In a sentence:
Western game perfection, thanks to an excellent balance between main and side quests supported by a fantastic story and legendary world design.
