level 1-79 is the part where you are smashing things like,one-shoting everything. After that when you reach lvl 80,you get passive ability to use mastery and things are more difficult to kill. Every class has its own mastery. As for demo, the meta form increases % dmg dealt for 1-79,and after lvl 80 your mastery kicks in. That formula changes to base dmg in % + mastery %. The only difference here is that mastery also increases dmg dealt by your minions and in your caster form. It is some kind of meta form for your meta form.
And yes,this is a great buff to your class. I think mastery and crit are your prio stats. Can't remember right now.