One feature that doesn't show up on either the downloadable or the GMail system - only on the Flash one - is the ability to post URLs to YouTube, Flickr, and Pi

One feature that doesn't show up on either the downloadable or the GMail system - only on the Flash one - is the ability to post URLs to YouTube, Flickr, and Picasa and have them automatically embedded in the chat window. I think it's an incredibly useful feature - similarly to how Facebook does that for posts - and I wish more IM clients began adding embed support.