Well I think there was a clear upgrade from the GTA III/Vice City/San Andreas era to the GTA IV/RDR era. The hardware was different (GTAIII era was PS2/Xbox, GTAIV was PS3/Xbox360), so it allowed them to upgrade their graphics.
Still, if you look at GTA III and compare it to San Andreas, the graphics and details got decidedly better from III to San Andreas.
And if you look at GTA IV, the graphics are definitely better in RDR. So I was expecting another improvement from RDR to GTAV.
Based on that trailer I'm not sure that it's an improvement over RDR, or even as good as RDR.
I'm sure next generation we will get another improvement though.