Are there specific types of wood that work better for an above-grade floor (namely, an attic)? I would imagine the lighter/less dense the material, the better it would be on the floor support. Or is this a non-issue?

Floorguy, read this and tell me what you think afterward. It's old science.

http://www.fpl.fs.fed.us/documnts/fplmisc/rpt736.pdf

I thought page 18 was pretty cool.