Some criticism:
"One of "mastercoin"'s problems is you can never have thin clients/SPV clients, etc. Each client has to be able to process and index the entire 8GB bitcoin blockchain in order to not be fooled or double-spent. Another issue is that mastercoin is centrally controlled by one party, all issued by one public key they call the "exodus address". There are quite a few technical issues with the whole approach mastercoin takes and it doesn't look to be feasible.
A more workable solution is presented by Freimarkets
http://freico.in/docs/freimarkets-v0.0.1.pdf by long-time rippler Jorge Timon and Mark Friedenbach. Freimarkets will allow transitive "ripple" transactions, but it will be a fork from the bitcoin blockchain.
For a practical colored-coin implementation that uses the bitcoin blockchain (any solution that does so must make certain sacrifices) but does not suffer the defects of mastercoin, see the "bitcoinx" project that Alex Mizrahi leads at
http://www.bitcoinx.org/. BitcoinX also does not support ripple-style transactions."
"Mastercoin is built as a layer on top of Bitcoin, so it doesn't have any specific transactions. Instead the upper level information is tucked into invalid inputs and outputs.
Since it's riding Bitcoin, it has all the drawbacks of Bitcoin: creepy slow, limited bandwidth due to fixed block size, arbitrary high transaction fee, centralized with two or three ASIC makers holding most of the network power including two Chinese companies...
And as a truly parasitic relationship it is going to harm a lot Bitcoin by spamming the blockchain and sucking all the bandwidth which will lead to further adjustments of the fees which will harm Mastercoin in turn...
The absolute most stupid thing is that Mastercoin allows anyone to mint custom currencies. This is going to be a mega spam party...
Mastercoin was built on top of Bitcoin because DaCoinMinster saw how alt coins are victim of ideological exclusion in the community and how Bitcoiners are inflexible fanatics. It's going to be a good learning for them.
We all know that Bitcoin needs small transactions to move off-chain if we don't want shit to hit the fan in the near future when adoption rate takes off. But instead of that Mastercoin brings more in-chain bloat at a whole new scale.
Mastercoin's success will be Bitcoin's Nemesis."