Bittrex:
"ERC23/223 increases security of smart contracts by ensuring that you can't send tokens to a contract that doesn't explicitly say that "i support tokens" by implementing a tokenFallback() function.  MGO decided to go this route and there was a miscommunication on that.  We didn't realize this and our smartcontract deposit addresses do not support it.  This is because when we starting using smartcontract deposit addresses, this wasn't around.  We made our contracts as generic as possible to support anything, but in this case, since we dont give an explicit yes, the contract wont send to us.  Other smaller exchanges have "no problem" supporting MGO cause they use regular ETH account addresses.  This is all fine and dandy when you have < 10k accounts, but the wallets won't scale to having infinite accounts, which is why we moved to smart contracts.

To support ERC223, will have to rewrite all our smart contract infrastructure AND replace everyone's addresses.  This is no small undertaking and we will do it when we get all the pieces in place.