The higher 100's digit has nothing to do with additional features -- arguably the 7XX series has far fewer features than the 5XX or some of the 3XX series - no hyperthreading, no 800 FSB, no SSE3. Of course, Intel marketing can just invent some features for these chips and solve the problem!
wbmw's original assertion was that you can compare performance across all models by looking at the XX digits -- this is false. The first digit doesn't even indicate the architecture, since IIRC, both Celeron Ms and Celeron P4s use the same leading digit, 3.
Petz