The campaign, being awesome to boot, offers a great way of getting used to the feel of the game before trying it out with friends or randoms. When it comes to the competitive multiplayer side of the game, the deep end is deeper than any other rts out there. There is multiple gaming houses with professional players trying to place high at tournaments for cash prizes. The "hardness" difficulty is endless as far as most people are concerned.
Having played RTS for 14 years I know top players from other games like AoE 2 trying to make it and not coming far. SC 2 simply have the biggest community with the fastest, most talented and hardest working players. It's as beautiful to watch as it is to play and improve at.
If you look to get a better feeling of what the game might entail I recommend teamliquid.net which offers professional player Streaming sessions and a great forum, news etc for free.