This has already been told, but a better support for mobiles and poor/ old rendering browsers is a must. Something as simple as using the useragent to display a nice, working and adapted login page would be a great start.
Another point would be reliability. Is there a way that you "priorize" oAuth requests in order to prevent any 5xx errors, for example? Arnaud. Twitoaster - http://twitoaster.com