The things that are wrong with K-9's interface are similar to the things wrong with Calibre's interface: they're powerful and capable, but completely defy convention, host an enormous amount of complexity, and can't be fixed piecemeal.
Things I dislike:
- The empty K9mail-errors virtual folder that I can't hide.
- The completely nonsensical sorting of folders (alphabetical by Folder Display Class, but there's no visual distinction between display classes, just an arbitrary point where your folder alphabetization resets).
- The quadrumvirate of Folder Display Class, Folder Sync Class, Folder Push Class, and Folder Notification Class are completely inscrutable.
- Even if you understand the class system, setting them requires one long press and 9 clicks per folder.
- I have to reach all the way to the top left of my phone to go from my inbox back to the list of folders, instead of following the common UI pattern of allowing me to pull in the folder list with an edge swipe. Tapping the top left is becoming more difficult with each increase in display size.
- I can't reliably refresh the folder list from my server, especially when it comes to special folder labels (Sent / Drafts / Spam / Trash / Archive)
So, it's mostly that the UI is pretty bad. Not unusably so, mind you, but neither is Emacs. The catch is, I just don't understand what benefit I'm getting from struggling with that interface.
And the issues are pretty endemic to the data model that K9 is built upon. To fix this, you'd need to completely re-think the interface and throw out the entire class system, which would raise another round of questions regarding how to handle selective mailbox synchronization, etc.
