I generally don't think of disc capacity as a "next gen" feature...Then again, CGI pointed out it is a 21 GB game.
Yeah, considering games in the PS1 era came on multiple discs, it isn't anything new. What's new is the fact that Sony opted to use Blu-ray for this gen, which means it looks that much better to be able to fit the bigger games on one disc. If they didn't use it, we'd be seeing the same complaints as with the 360, with multiple discs. Game sizes aren't getting any smaller, so if it needs multiple discs, then I don't see the problem, as it isn't a problem getting up and swapping discs. The only problem would be if you needed to do it every single time you wanted to go back to an older area (didn't Star Ocean 4 on 360 have this problem; I remember some game on 360 did), which doesn't seem to be the case with Rage.