Just because I want to play as a monk in Diablo 3 doesn't mean that it'd make a compelling class choice in WoW! Check question #2 for hero class talk. Honestly, any excuse to talk about the monk is a good excuse. When is Diablo 3 coming out again?
Here's a newbie question: What the heck is the global cooldown for?
Essentially, the global cooldown exists to prevent spamming of abilities. Think of all the instant spells you have at your disposal. Now imagine that there's no limit to the number of times you can hit them in a second's time. You could macro as many Ice Lances as you could cast with a full mana bar and unload on the nearest whatever.
Now, of course that'd make for some decidedly unfun PvP, but think about it in terms of server resources for a moment. The server has to keep track of all of the requests you're making of it, from "use this item" to "cast this spell." If there were no hard limit to how many of those requests you could make at a time, the servers would be under dire stress 100% of the time. It'd be like old times in Lagforge or Lagrimmar ... or the AQ gate event!
The GCD also helps with game balance. "I can do X damage in one GCD" is a phrase we hear quite a bit, and it helps the developers when they can say, "We expect an elemental shaman to be able to do this many thousand damage in a single GCD" instead of, say, drinking themselves into an early grave.