Complexity. We have 10x more code, 10x more components, and 10x more layers than we need. If the initial architecture had been dirt-simple, I don't think the site would have had so much trouble scaling or staying up. But, of course, removing complexity without breaking things takes longer than adding it in the first place.

The other big problem is operations — many steps are done manually which should be done with tools like chef or puppet. When you have a lot of manual steps in your deploy process, it makes the whole system harder to scale, test, modify, and keep running. "Devops" has become a buzzword but it's definitely needed here.