Using a 56kb dial-up connection, I really notice failures in the web applications which assume network state will persist, or which have, under other circumstances, minor usability flaws.
Take JetBlue‘s ticketing process for example. All of the pages are POSTs, and don’t persist in cache, so using the browser’s back button could reissue the transaction. This is not something you want when you’re spending $800 on non-refundable tickets. This failure is aggravated when there’s a problem with the credit card, either because their systems fail or you mistype a number; they tell you to go back, but it’s not that easy. And these are nothing compared to your line dropping in the middle of a transaction like picking seats.
Song shows you the seats available on the flight before you begin the purchase, so if that’s important, you can eliminate flights with a paucity of seats.