While it would probably be impossible to implement in a way that actually worked, it would be super-awesome if a password manager could auto-detect a website or service's password requirements and then automatically generate the most complex password that met those requirements. I'm so tired of having to decipher the random pop-ups and dialogs informing me that my password doesn't meet whatever the random criteria are and then go fiddle with the password recipe.
Like I said, it's probably not feasible, but a kid can dream, right?
All of that said, if you are not using a password manager, you really should. I like 1Password myself, but your mileage may vary.