1. it's ugly
From a programmer's perspective, the UI is massively different. You really need to go through and update all your artwork and customizations to match. The size of the navigation bar changed (and it's translucent by default) so you also need to rearrange a lot of your views.
There's also a lot of minor things where maybe you were doing it wrong before but it didn't matter... and now it breaks. Or deprecated stuff (which can accumulate for apps that have been around for a while).