A few suggestions

MusicXML allows for 

• figured bass texts. These don't seem to be imported at all!
• horizontal frames which allow for text to be inserted at the beginning of a system while still on that system line as opposed to above it or below it.
• courtesy clef/key-sig/time-sig as being on/off depending on settings. These seem to import as always on and need manually shutting off when importing from MusicXML per each instance of their occurrences. NoteFlight could probably take into consideration to have them showing or not if this information is in the xml data. 
• circled text instead of only "rehearsal marks" as being rectangular. That would be nice for certain situations like guitar fingering etc. 

A few upgrades and NoteFlight  would be that much better! Thanks for the free account.