Which components actually make a difference?

hubs also, i've held good quality hubs and cheap hubs and spun them just holding the axles and pushing the hub body and there is a big differenece in how long they spin, novatec's being the best value if anyone's asking, they spin for almost as long as the expensive ones but are a fraction of the price

This test is meaningless, as is spinning a wheel in a wheel jig or in the forks of an upturned bike. This is because bearings behave differently under load.

Skaters will recognise that when you fit your wheels with new, posh bearings and you spin the wheel they come to a stop pretty quick, but put the board on the ground, apply your 70 kilos and gravity, push off and they will fly!