No noise in the blue channel at ISO 100, no fear off using auto ISO, much sharper image, better color separation, more consistent exposure, etc.
To put things into perspective: Using auto iso 100-800 on crop seems fine to me, the sharpness of an image is more dependent on the lens (think ff+cheap lens or crop+expensive lens, not $1000 vs. $5000), and exposure in eval at least on the 6d seems to be more erratic than on 60d and has nothing to do with the sensor - the dynamic range is about the same.
I do agree about color & noise as I wrote above, though my answer is intended for roguewave and he was asking about downsized shots, so the even iso performance isn't paramount for him.
Now, if the IQ is substantially better as most posters claim, that would be an important factor. Else, if it's a slight difference that most people won't notice without pixel peeping, then I can live with that.
Crop sensor performance of Canon is a bit beyond Nikon and the 70d's 20mp has't advanced much vs. 18mp, I guess that makes many people bash it in *relative* terms - but in *absolute* terms it's really fine, I've been using the 60d for 2.5 years an 150k shots and the only real "no go" area is shooting motion indoors.
Last not least if people spend a hilarious amount of money on a gadget I'd wager to say it's tempting to rationalize a fun purchase (and the 6d has great iq) as essential even when in many situations crop would deliver the same result for standard print/view sizes.
Tokina 11-16, Canon 17-55 f2.8, Canon 17-40, Canon 24-105, Sigma 30 f1.4, Canon 85 f1.4, Canon 70-200 f4 IS, and a few more :-). Depending on which camera I get, I may sell the crop lenses and get Sigma 35 f1.4, Tamron 24-70 f2.8, or both.
I'm also not a big fan of the sell & buy game and rather stick to what I have and purchase other things that are also important (esp. lighting gear (flashes, diffusers), but also monitor, color calibration, tripod, filters, printer, software ... repairs!). For crop your 11-16, 17-55, 70-200 should indeed about cover it, the 17-40 & 24-105 are really ff lenses in sharpness & zoom range even if they add weather sealing.