Flickr is, for the most part, a pretty usable web app with a decent UI, but one thing that's always bugged the crap out of me is the "click here to add a description" area under the image on a photo page. It doesn't appear until you mouse over it, so it's completely non-intuitive unless you already expect it to be there, and even though I've been using Flickr for over a year I still almost always start typing my image description into the "Add your comment" textarea, which is always visible. I hate that.