In front hubs it is less noticable, but the real diff comes in the rears. On the one end of the spectrum you have the Chris King hubs. They use a completely different freewheeling system inside and have a 5 yr replacement warranty.
On the lower end you have the entry level shimanos. They for the most part are similar to the rest of the shimano line and many other mfgs, differing only in material quality control. They charachteristicly suffer in the seals department, being prone to low service intervals. If maintained they do get the job done.
Then there is the sealed cartridge variety. They can vary from the basic yet highly dependable SunRace Juju all the way up to Hope and American Classic. The Juju design is a little heavier than the AC or hope but is simple and you can almost forget its there.
Cartridge hubs can be deceptive however. The cartridge bearings in most hubs are of less than impressive quality(I am used to good bearings
) but they can be replaced easily with industrial grade. Even still, a cheap cartridge that has been greased propperly(factory grease is often suspect) will still last in the thousands of miles before needing work.