There are some systems where blockchain technology will be really useful (#1 for me is zerocash for non-government-sanctioned transactions, but that probably won't be ready for real use until 2015; not the least of which because we need more secure wallets and other systems to handle a fully anonymous irrevocable currency than even what we have for bitcoin today).

I don't think it's going to take over everything. One, a decentralized network of distinct currencies with an exchange is a better route to decentralization than centrally-coordinated protocol-decentralized currencies for a variety of reasons (performance, ease of modeling, ...); two, a lot of things are inherently centralized.

On OP's third point about reversibility being a feature -- the solution to that is to leave funds in escrow. So, you can do $100 transactions as long as you have $100 at-risk in an escrow transaction, and can do an aggregate volume of transactions up to the amount you have on deposit. Maybe some more sophisticated risk modeling there, too (older accounts, lower risk transactions, etc. take less of your deposit balance).