There's a few extra stages in the 1400 and 1500's YPAO. The 1XXX series have full parametric equalization (which basically smoothes out the frequency response in your room), a feature that can have as big an impact on sound quality as anything. But I wouldn't buy the receiver for this feature alone (actually I did)...in my old house, it didn't make a difference at all...in my new house it does make a slight difference, but not worth the extra $300 IMO...I believe in some rooms it could be a huge factor.
Anyway, the RX-V750, RX-V650, and HTR-5760 get my votes as best value receiver on the market right now. If you've got the cash, or want a bit more you get more options. Don't put yourself in the poorhouse doing it though.