This info is outdated. Here's the answer for anyone interested in getting SC2:
You are not restricted to 4 maps or to terran only: You have full access to all races, maps and mods in "custom game", and all games in the arcade.
However, you still cannot play ranked ladder games and you can only play against easiest or easier AI in single player scenarios (though you can get around this by starting a "co-op vs AI" multiplayer game, setting the game's difficulty higher, adding an AI and playing against them).
You are also still restricted to the first 5 missions of the Terran campaign, and only the first two Terran challenge missions.
You also cannot test or publish your own maps, though you can make them.
Your level is capped at 15 (5 for each race), preventing you from unlocking any skins and most decals (though these aren't important features).