I agree that the navigation is pretty well done (in my MX-5 ND2), but one horrendous thing is this Gracenote thing that tries to guess music tags and covert art on external media (i.e. USB drive), but in the process completely ignores AND mangles the tags I've carefully put in my Vorbis files. Of course, you can't disable it.

I also think that the whole interface is quite sluggish in general.