Libraries are a problem - it's difficult to switch unless all your dependencies are Python 3 compatible. The only ways to fix it are to either use an alternative, or perhaps to fork or contribute patches to the project - both of which take up valuable time when your code is still functional under 2.7.
I'm currently creating 2.7/3.3+ compatible code in whatever I create, but I'm still deploying to 2.7 hosts. For most of my projects this is due to just one package which has some Python 3 related bugs, so hopefully I can just flick the switch soon and use 3.4.
It's only easy when your project depends on a few popular packages (eg. the top 200 where 86% are currently Python 3 compatible [1]), it's much more difficult if you've got a long list of dependencies.