It drives me nuts to see children who are still in strollers in spite of the fact that they are far too big. Over the summer, I've seen a girl who appeared to be about 10, knees in her chin, being pushed by her parents while eating ice cream and reading "Little House on the Prarie", among many other similar offenders.
Let them grow up!!

Another one that's right up there for me is all of this referrences to "moms" on all of the activity and advice parenting sites and boards. I'm a working mom with a stay-at-home dad, but 3/4 of what's out there is billed for or limited to moms!
What's up with that?!

My biggest pet peeve is mommies who ignore their children's plea's of "Mommy lookit, Mommy, Mommy, Mommy, Mommy lookit, Mommy, Mommy". I know we are over worked and underpaid, but a simple word of recognition can go a long way in keeping harmony.(and people around you will save their dirty looks for someone else)