I meant mmorpg srry about that XD

Do you mean he game didnt have single player and was just multiplayer with classes?
If so heres what would happen:

If pvp was enabled then the gods with all their amazing armor and swords would rule the game and kill noobs for fun. Taking over the world and destroying other people's stuff and taking their items. The game would be no fun and the evil tyrants would rule it.

If pvp wasn't enabled then people would steal other people stuff, there woulld be so much lag caused by the millions of people that the fps was 1 or 2, 3 if you got lucky.
it would be greifers galore and all diamonds would dissapear withing a week from the million or so miners.

Eh, there'd probably be classes. So half of the players would pick the most imbalanced class, while everyone else would hate it and complained to Mojang to nerf said class.

MostlyMenOnlineRolePlayingGirls

Possibly meant MMORPG?

It already is an mmo of sorts isn't it? Wikipedia says this:
A massively multiplayer online game (also called MMO and MMOG) is a multiplayer video game which is capable of supporting large numbers of players simultaneously.
Although there is single player, minecraft is definitely supports large numbers of players simultaneously.

Well Minecraft is an MMO, ever heard of multiplayer? Do you mean RPG?