Ismael Ghalimi hits the nail on the head with the post - Office 2.0 Interoperability. I'm one of the victims of what I'd call, "Oh, I forgot what login I entered for this web service" syndrome. Sometimes I don't trust a lot of web services and enter an alternate login e-mail address. It becomes nearly impossible to remember them all, and having a "Remember Password" isn't a great idea when several people use the same PC terminal. (as pointed out in the post)
This is where Google's services score over the other. A Gmail address is sufficient for accesing all Google affiliated services, right from Google Reader to Orkut. Like the post says, it is a very complex problem to solve, given the number of e-mail service providers. What I'd suggest is something similar to a feature of Zoho Writer: There is a "Post to your blog" option with a list of popular blog service providers, it would be great if genuine web services offer a "Sign in with your X email account" where X would be an e-mail service like Yahoo, or Gmail. The web service would remember what X is, but not the username and password. A lot safer.
That system has a major let down, though.
I have a primary e-mail account and a secondary e-mail account, both with Gmail, for its user-friendliness. If I select Gmail as my service, on my second visit, the web service will remember that I used a Gmail account to sign in. I'll have a problem remembering which of the two addresses I used. It's Ok in my case because I have just two accounts with the same provider. A lot of people have more than two accounts and they'll still have trouble signing in.
Single Sign on, though a complex issue is, as much as I could say, the best way Office 2.0 interoperability can be acheived. After all, compatibility between two services is an important feature that people look for in any service.