Ok, I played the campaign. I played about a week of mulitplayer (silver league), and I am now afraid to jump in. BUT: 
 
Man, those Tower Defense maps are fun! I played Char Tower Defense (with an interesting power relay/distribution mechanic), and another one. Just create a custom multiplayer game and search for popular tower defense maps. You can play most of them alone if you want. 
 
Yeah, I know pretty pointless post, I just wanted to remind people how much from the community is in there now.