For many, the flexibility of Bitcoin in terms of what sorts of scripts can be executed with transactions is much of the appeal. I'm of the opinion, however, that it represents Bitcoin's greatest weakness.<p>In order for bitcoins to be useful for money, it must be possible to determine how many bitcoins I have, or, more exactly, how many I have available to spend. The fact that there is a language associated with bitcoin redemption, and the redemption happens at the time of the creation of the outbound transaction makes this very difficult to determine.<p>So, more clearly, if I want to spend some coins that have been nominally sent to me, at the time that I attempt to send them to another address, I have to have all the information necessary to redeem the coins that were sent to me, for each instance of an incoming transaction to my address.<p>If this sounds complicated, it is because it is. Ethereum, one of the many post-Bitcoin cryptocurrencies, does a much better job of this on the pure currency side -- spending coins at an address to which I have the private key is just a matter of saying how many coins I want to send and where. Ethereum, unfortunately (in my mind) complicates this with a much more sophisticated language that lives in a side-chain.<p>I think the ultimate Bitcoin successor (or modifications to the Bitcoin protocol itself) will have to prune down the scope of allowed transactions, rather than expanding it. This isn't just a question of the "my grandmother won't be able use it" -- although having an unspendable incoming transaction from the "publishers bitclearing house" for a million bitcoins will definitely be a bit offputting.<p>In the end, even for a sophisticated user, it will be nearly impossible to present a view of how many bitcoins you have available to spend. This is why the core devs have been very reluctant to allow different kinds of scripts; multisig is the first real expansion.<p>Colored coins, while kind of fun, live outside the blockchain, so don't really bother me. It's basically the equivalent of a (for example) car saying "anyone who holds a dollar bill with serial number xxxxxx from year yyyy can start the engine", in a world where it is impossible to forge dollar bills (as is the case with bitcoins).