Did they attempt to do anything to deal with DOS-attacks? The problem is that if you blacklist the input address, someone can just go create another input address (even passing through another instance of the mixer). If they are sharing a single blacklist coin-wide, then it is not a decentralized coin (i.e. some pools could lie and cause addresses to become unspendable). Etc, etc, etc.
A cost of 0.1 DRK was added that is not returned but distributed to the miners if the client misbehaves, so organizing a dos could become a costly endeavor. Again, possible I got it wrong. Someone else please verify.
I am thinking DOS can still occur free-of-charge in the signing of the outputs.