The website scans through your Twitter feed and then shows you, one by one, all the tweets that nobody favorited or retweeted, one tweet at a time. The tweets zoom toward you slowly, inescapable in their awfulness, one after another, ceaselessly. Prepare to shake your head at your repeated Twitter defeats and wonder where you went so, so wrong.
You can try out Sad Tweets here. Unfortunately, you can view only your own Twitter feed’s saddest tweets at the moment; hopefully soon you will be able to view the saddest tweets of your friends and colleagues, so that you can mock them and make yourself feel a little better about your own failures. (Or, you know, commiserate with them.)
I’d guess that somewhere around half my tweets received neither retweet nor favorite; this would mean that I’d have to sit through around 25,000 of the darn things. No thanks. They weren’t all that wonderful to begin with.