I don't think we have exacts (e.g., numbers) but Nephalem Valor is a stacking buff you get for killing Champions, Uniques, Bosses, etc. enemies. This bonus is lost if you leave the game or if you change your current skills (e.g., swap out a skill for another or select a different rune).
Basically, the system is that all level 60 characters of a certain class will have the same stats and toolbox. Your diversity is in your equipment (which will be 90%+ of your "stats" anyway) and your skills you're using. The skill combinations are somewhere in the upper millions, I believe. Essentially, Blizzard moved your final build planning from the start of the game to the end of the game. It's no longer front-loaded.
I like the sound of that generally, though losing the buff by leaving the game is a bit... meh. people "devoted" will just have their PC's running 24/7 then I guess.