Impersonation by Gravatar
There's a discussion going on at work regarding the potential for impersonation using Gravatar. There is some merit to this concern, though I consider it a little flimsy. Generally speaking, Gravatar is a service that ties a picture to your email address. A malicious person with knowledge of that email address, combined with a Gravatar-enabled service that allows user-generated content but does not verify email address ownership, makes for trivially easy spoofing.
Gravatar solves a problem: social sites across the web require us to create accounts and specify an avatar. For most people, the email address is the common thread running through all these accounts. If your goal is to provide a central avatar service, it makes sense to tie your service to email addresses. As a consumer of this service, if your site lets people provide an email address and you don't verify ownership of that address, you're making it very easy for people to spoof each other.
I don't get too worked up about this possibility. A determined person will impersonate whether or not Gravatar exists. Given a semi-famous web personality or a Googlable name, anyone could register a fake email address and use that account to create social accounts. Combine that with readily-available photos and the illusion is complete, with or without Gravatar. A potential employer Googles and finds email@example.com making inflammatory comments on GigaOM, and the user photo matches up. Next resume, please.
As I see it, Gravatar is a valuable service, made more valuable when we can verify identity. To encourage participation on our sites, we have to lower the barriers to entry. As the people creating the modern web, we have to reconcile these these two sometimes-competing aspects of the social web.