- I really should have backed up my old layout before I started, and
- Not all themes give you access to the tools you've become accustomed to using for the last decade and more.
The most inconvenient change: I can no longer create a new post, or edit an existing post, right from my blog. Now I have to go to the Blogger dashboard and do it all from there.
I've since changed it back to a less cool and snazzy theme that does actually include the tools I want. Take THAT, hipster UI designers!