Blockchain bloat, which is only twice as big as Bitcoin's, I've grown tired of simply reposting what has already been said, 1,000,000 times.
I wouldn't use word "only" when something is twice as big while having 1/100th of transactions. Even if you reduce it by 50-70% it will not scale for mainstream. Bitcoin based coins won't either in their current state, but there is a known solution for Bitcoin based coins - mini-blockchain.
You understand that if Darkcoin is mixing coins to any degree it will lead to blockchain bloat that meets or exceeds the levels of CryptoNote coins? Good, just wanted to make sure you were aware.