Why does a subpanel need separate ground and neutral?
Bonded ground/neutral

If you have the neutral and ground bonded at a subpanel, then you'll get neutral return current through the ground wire back to the main panel (since there are now multiple paths). Even worse, as @Tester101 points out, if the neutral ever has a fault, everything will continue to work but you'll have all the current on the ground, which also means that you can now be electrocuted by touching the panel chassis, for example. 

The ground and neutral must be bonded only at one place (in the main panel) to avoid this. 

Subpanel main breaker

A main breaker on a sub-panel is not necessary because this is in the same building (if you are in a different building then NEC 225.31, 225.32, 225.33 apply). That said, having a main breaker in the sub-panel is also acceptable.

For whatever reason (economy of scale, I guess) "main" panels are often sold cheaper. I recently purchased a small 12-breaker panel (which included a main breaker and a couple 15A breakers) for almost half the price of a similarly-sized sub-panel (which didn't include any breakers). It makes absolutely no difference to wiring, you just have to be sure to take out the ground/neutral bonding screw/bar (if pre-installed).

Subpanel ground

The sub panel must absolutely have ground back to the main panel, and it must be appropriately sized for the current rating of the panel (as in, it's the same as or larger size than the hot/neutral wiring). This is regardless of being in the same structure or not.