What I find really iritating with Twitter and Facebook logins is that the rights an app asks for (e.g. post tweets, read DMs) is an all-or-nothing proposition.
It should be more configurable at the point of login so that you can refuse to grant something that a 3rd part app is asking (e.g. often I don't want 3rd party apps sending tweets as me), and let the app figure out if thats ok.