RE: seeds with a lot of entropy:
Every time I look at the academic literature on passwords/passphrases, I get more depressed about the feasibility of either giving users a secure passphrase that they will remember or getting a secure passphrase from them. I fear there will be a lot of lost coins if "brain wallets" get popular.
E.g. this 2012 paper: http://dl.acm.org/citation.cfm?doid=2335356.2335366
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