Hola buenas.
I never used maemo 4 but i can get the ball rolling with what i know.
Nokia maps yes works well.. I think some ppl are working on 3d and voice support... But checkout sygic.

Java yes.. Jre works.. Some ppl are working on port j2me now.

Everything else.. I doubt works
but n900 has its own perks and very its a compitent phone..
U'll love it
saludos desde australia.