Hardwood flooring in many rooms

[Bob10]

You can always put door sweeps on the bottom of them. Come in brass that would dress them up and serve a purpose.

That too, in my opinion just about anything other than making it so high someone falls or smacks a toe.

[MstrCarpenter]

Thanks, I understand how you recommend doing the thresholds.  It seems that this way would solve several problems for me.  It would allow me to do one room at a time and take a break in between without leaving too much of a mess at the doorway, it would hide any gaps between the two rooms, and it would raise the floor 3/8 inch to help close the gap below the door without creating a tripping hazard.  Would you just glue the threshold and plywood filler down or face nail too?

I usually just face nail where I can't get the nailer except for areas that get a lot of traffic (i.e. stair landings and doorways). Those areas get a couple of thick beads of fresh construction adhesive, pushed down firmly in place, then some 15ga. finish away from the highest traffic spots. Try not to step on it for a day or so. Screws are best for the filler. If you use marble at the bath, don't use construction adhesive,  It will take a while but the oils in it WILL telescope through. Use a mortar product like modified thin-set instead.

[Cooler]

I face nailed the thresholds with 15 gage finish nails. You could also use small head finish screws and that might be a better choice.  Putty over the nail/screw holes or make matching face plugs.  I used three coats of clear oil-based poly, but whatever finish you are using on the flooring would work. The thresholds' finishes have held up remarkably well and I suspect that in most cases they get stepped over rather than stepped on.