Mainly two things. It doesn't thread conversations in any discernible manner (and if there's some magic combination of the micro-settings that does result in a readable, ordered conversation view, I don't have the time to try all the permutations possible to try and stumble into it; it's not helped by having indecipherable labels for those options. Sensible defaults are 10000x better) and it hangs on refreshing the timeline — bad enough that the Android frozen-app-executioner pops up volunteering to kill it.

And there's a lot of little UI inconsistencies, strings that need fixing, etc. But the functional stuff takes priority.