If you're wondering why you can't just download a map and play it, go to this thread to read about why:
Blizzard devs have supplied us with a wonderful map editor, with a lot of new options and possibilities, but whether it's business or incompetence, we are denied a simple or easy way to play them.
As it is, the hosting systems of WarCraft III and StarCraft 1 are far superior to that of SC2.
There's an eerie silence in both the US and EU communities from the devs and community managers, no one wants to say why things are the way they are. My guess would be one of the more popular theories, that Blizzard Activision is forced to be able to control all uploaded and custom work, in order to keep the rating of their game and to increase sales.
In the process, they are alienating customers, killing their reputation of being a brilliant developing company.
I am sad to say that Blizzard seem to have failed to take a stand against game ratings and have turned corporate; now more than ever.
My faith in this company has run out, I will never buy a game untested or untried from this company again and I feel that this promotes pirating games rather than paying for them.