The main difference is that instead of a password that was chosen by you, a secret key generated by Twitter gives them access to your account. They can still add or remove followers, send DMs on your behalf, or post tweets, or replies. When you give an app permission with OAuth they get to be you on Twitter, exactly as if you had given them your password.
True, each app gets a different secret key, so you can de-authorize one app without de-authorizing all. That's the advantage that OAuth gives users. Sure, it has some value. But it's a convenience, it doesn't add security.
See also: Jon Udell on a cost of OAuth.