I don't see why, really; after 7 years we're all aware of what the current gen is capable of, and it's still going to be a significant improvement over GTA4.
It'll be the people who have got themselves caught-up in the next-gen who aren't impressed with GTA5, but it's a case of managing expectations really.
That's exactly it. There's a lot of people that will be caught up already. And Rockstar aren't really managing expectations. We've seen a lot of shots that I struggle to believe aren't touched up. And the features on their website use a lot of pre-rendered graphics.
Not that I really mind, that's their problem. I'll be enjoying the game!