The reason I don't use LastPass is because it is proprietary. I use KeePassX over firefox because it has better security, can store associated details (Comments field which I can put, for example, what my secret was (I randomly generate those too) and so on), and can easily be used for passwords that are not for only web based stuff.
The most important detail would be the fact that passwords saved in browsers are only useful in the browser, not for apps etc.
As another random example, firefox couldn't save, say, the passwords of sites I ssh into or the irc keys I need to authenticate myself on various irc servers.
Once you set up a master password then firefox encrypts them in the password database. That button is still there, but using it requires the master password before actually it shows you the stored encrypted password.
1Password has abstracted the browser away from me, so I can use Chrome, FF, or Safari with ease on Win or Mac.
Dropbox integration seemed nice, but I'm now strongly considering moving my keychain to a different service.