With little kids it is very hard to sit down and rationally discuss the pros and cons of their behavior. Reason is boring to a toddler, preschooler, etc. They like, and need things that grab their attention. A swfit smak in the butt can really do that without actually hurting them. Hitting in the face is no good. It hurts.
I believe the line between discipline and abuse is in the anger. A dad who grabs his five year old by the arm and smacks his rear while taking him back accross the street he just crossed against the dad's safety rules, while reminding him he is not to do that again, is acceptable to me. But a dad who does all that and them spanks him and slaps his face and tells him to "stop crying or I'll give you something to cry about" just because he's pissed off about being disobeyed or inconvenienced is just an abusive asshole.
It is also worth noting that many kids are so abused an screwed up by their parents without ever having a hand laid on them.
Another thing that is a big mistake to me is being incapable of doing anything that may cause your child the slightest discomfort or disappointment. Some parents are the complete opposite end of the spectrum and that can do even more damage than a poorly judged smack.
It would be so nice if you could just say you should never touch your kid. But parenting is a hard and takes subtle observational skills and jugement, to do it well anyway.
)
The last time I got smacked was when my Mom was chasing my brother and I around the house with the famous wooden spoon. She finally got ahold of us and broke the spoon on our butts. We wern't quite sure how to handle that situation, a moment of silence passed, then we all started to laugh uncontrollably. That was the last time I got smacked. When the smacker brakes, it's time to give it up. 
