I know two of the biggest advantages of using open ID is that users don't have to remember yet another user name/ password and that you don't have to worry about confirmation email messages ending up in spam or not being sent at all. I'm wondering what the drawbacks are for using Open ID? Are they're any?
Well the biggest drawback is that is hellishly complex and implemented slightly differently on each provider. Some return email addresses via sreg, some via ax and some not at all. At the base of it all it gives you is authentication, meaning it lets you know that somebody is who they said they are.
There is also the problem of trust.
Say I have an openid named http://waffles.com it may tell you that my email address is [email protected] if you trust the openid provider all is fine, but keep in mind that anyone can run an openid server. So you end up whitelisting providers which makes things more complicated.
The other big problem is that some non-savvy users are afraid of open id and do not understand it, they are used to traditional sign up / on and will not consider anything else.
There are some security implications. See for example the accepted answer to "Are there any security risks associated with me using OpenID as the authentication method on my site?" (on Stack Overflow).
Well, I can tell you one huge problem with OpenID -- Stack Exchange seems to have dropped support for it. A month or so ago, I found myself unable to log in using my 1id login. That problem got fixed, but as of today, I found myself unable to sign up on other stack exchange properties (like this one).
I finally created a conventional sign up on Stack Exchange just so I could post to meta.