[Dectilon]

Basically it goes like this for the sc2-campaign:

Casual: The enemy doesn't really build units and have half health compared to you.
Normal: Same AI as casual but normal health.
Hard: The enemy actually builds stuff, if you want a decent challenge this is the right difficulty level.
Brutal: Ohmygodwhydotheykeepcoming?!?

There's plenty of stuff that annoys me in the campaign (like kerrigan going from a pretty good villain in BW to... a whiny nihilist? And the most drawn-out Fox News-joke in history.), but it's good enough that you won't get bored if you play on hard.