So after doing a little browsing through Threadless's "rival" Design By Humans, I've come to the conclusion that most of what they print would be rejected designs here.
They also seem to have a certain fetish for covering up every last inch of the t-shirt, which, in my opinion, just screams "WHOA! LOOK AT ME!!!". I'm not saying some Threadless shirts don't do that, just not 99% of them.

I also think we're cheaper.
By a lot.