Providers should allow others providers OpenID to be used on their sites
As an example, I've registered with google gmail, so I have an OpenId account from google. Let say I want a Yahoo email account, and I want to use my OpenId from gmail, well I don't see any option for that from yahoo.
Some providers give the option to use existing OpenID but hidden in very small print. Competition I guess.
All these defeat the simplicity mention by openid in the first place.