Chuck Westfall is quoted as saying that the optics are as good if not better than the 24-70L. Can't see it somehow, but if they've managed that then it would be a serious first...
Well, I'm actually surprised that Canon hasn't released more noticeably improved lenses in the past few years than we've seen. Okay, the 24-70 and 16-35 may be said to be such design wins over the 28-70 and 17-35 respectively, but still ...
Optics have undergone a few revolutions lately, and modern CAD/CAM should yield significant benefits towards actually making noticeably better lenses, versus old hat design methods.
The 24-105 has two things going for it:
1) It's slower. That makes it easier to achieve higher quality optics, right?
2) It's far, far newer in a rapid-moving technological world.
Whether this is enough to actually make it better optically than the 24-70 f/2.8 remains to be seen, but I'd be surprised if it at least wasn't close.