NAT does a little more than just hide you. It is still fairly weak security, but imagine this scenario.

You have a flimsy pathetic lock on your door
None of your neighbors have any locks

Even breaking your pathetic lock will take time and a small amount of expertise. It is unlikely anybody would bother. That being said, the average user probably gets portscanned about a dozen times a day. If you leave yourself entirely unprotected, you will probably get somebody probing or attacking your system every day Damage will occur extremely rarely however. In fact, you will probably never experience any damage.

"If the last number of that address is a 0 (as in 192.168.1.0), then it's not doing anything, right? "
Everything is fine then. That is the proper setting for most people.

-Biscuits