I'm personally comfortable with doing such things in public as sitting on a partners' lap or kissing them (keep in mind I'm French, so what I call "kissing" you call "French kissing"). I grew up where both happen in public regularly and aren't weirder than holding hands or having your arm over one's shoulder.
My husband is comfortable with doing these things in public too, my boyfriend less so. Last week (he came to visit from the US, yay!) I wanted to sit on his lap when we were waiting for the shuttle and he told me he didn't think that was appropriate. I was pretty surprised as it seems pretty mild to me, certainly less intimate than hugging for instance since you're less in contact.
When I kissed him later he said he was worried people would ask us to get a room. Once again, I was surprised as it doesn't seem weird at all to me to kiss if you're a couple.
I had a friend tell me he would be fine with me coming along with my boyfriend to an event as long as we weren't all over each other. I assumed he meant things like groping and inappropriate comments, but it seems just kissing was included too.
So, the rule of "everything you'd do in front of your parents or in public" isn't that clear-cut. I French kissed my husband (then boyfriend) in front of my whole family (parents, siblings, grandfather) the first time he met them over supper. And not just once, I did it lots and nobody found it weird. I've seen my parents kiss and snuggle and I would have found it weirder if they didn't.
I guess what I mean is that it's good to check your partners' comfort level with them. If you tell me "whatever you'd be comfortable doing in public or in front of your parents" it basically only excludes sex. For other people, even holding hands would be too much. So being specific and not assuming they have the same views on what's appropriate or not would help.