I just completed my hardwood floors this weekend. I used Minwax oil based
polyurethane, 2 coats. After the 1st coat, I followed the directions,
sanded, vacuumed, and wiped with mineral spirits. I was overall pleased,
but it's not real smooth, and lots of little bumps, which I presume is dust
paticles. Is there another step in smoothing out the final coat, maybe
super fine steel wool?

No matter what the can says, double the # of coats in order to have an
absolutely fine and clear finish that will last and last. Sounds like
you either didn't seal the room while working on it. Or didn't sweep
before the first vacuuming. Or you in some way shook the can. Here's
how I did it: Floor sanded: once with course; twice with fine; once
with extra fine. Hand wiped with damp lintless rags that can be thrown
out. vacuumed and let dry overnight. Open the windows to vent in the
morning after sunrise. 1st coat of finish. Close windows at dusk to
prevent moisture carriage. Hand sand with 000 or 0000 steel wool. Rub
with barely damp rag to pick up the heavy grit. Vacuum twice. Again,
in morning open windows and second finish coat doing the same
proceedure. I did this for 14 coats. It never needed polishing, just
light dusting. It lasted 20 yrs and still looked like glass. I used
Martin Seynour Clear Varnish.

Minwax is one of the best on the market if not THE best. Sounds like
you either didn't clean completely between coats or you shook the can
some how and air bubbles in it. Here's what I did: First seal the
room. Machine sanded with coarse and then with fine and then extra
fine grit. Damp rubbed the bare wood. Sweep the wood dust up for
later use. Vacuumed and closed the windows that night. After sunrise,
when the moisture in the air is gone, open windows to vent. Let first
coat dry over night. Windows closed. open them in morning. Hand sand
with 000 or 0000 steel wool, again hand damp rub and vacuum twice.
Keep a roll of masking tape on your wrist so you can mark the nicks and
scratches. After the 2nd vacuuming, go back and paste those spots
with the saved wood dust. Apply second finish coat closing the windows
as you pass them. Next morning, vent and hand sand, damp rub, vac
twice, then apply third coat. Do at least 4 coats in order to have a
good strong finish. I did 14 coats on mine, tinting in different
rooms to set an atmosphere. Transparent pink in bdrms for warmth.
Blotched silver in study. Clear in lvrnm. Red in dngrm. All floors
needed was a light dusting. They still looked like glass 20 yrs later.