With the initial implementation of transposable chords in the latest test release (r911), MuseScore is steadily becoming the perfect lead sheet editor.
There is one killer feature dough that is missing: playing the chords (i.e. chordnames) during MIDI playback. This could make a huge difference when transcribing or correcting a lead sheet.
This feature would be a pretty nice differentiator from commercial music notation software such as Sibelius, because the chords are not being played.