I think for such 3D map you should really focus on usability - eg, make a good zoom/rotation by default, add keyboard support, easy rotation (maybe 90 degrees rotation), etc. Basically try using it yourself - it should take little time and effort to see the overview of some areas, so it will be appealing for other players
Don't use right mouse button pressing, as it conflicts with some popular browser extensions like FireGestures.
One of the features I would've really liked in such 3D map is a level-filter (lower underrail, underpassages, caves, etc.) so you can easily switch between various levels in 2D-ish fashion