The characters, the story missions, the climbing on rooftops and jumping on zip lines, the sabotage side activities, the cars, the music, the wanton destruction...
the shooting's not that great and sometimes the climbing gets bunged
not the greatest game in the world, but it has style to spare and isn't bogged down with having to go on dates or wade through an ocean of copy/pasted creatures
it's better than GTAIV and Crackdown 2, but not as good as RDR