All those "like a girl" comments damage girls' perceptions of themselves, and influence how others treat them. It's a bad thing to perpetuate.
I remember reading an article a while back about "throwing like a girl." It was making the point that boys and girls alike often start out throwing in inefficient ways. But what happens is that with boys, people coach them with specifics so they can improve their form, because it's part of "boy culture." Girls, on the other hand, are often told "you throw like a girl" and they're dismissed.
“It's what everybody knows the problem as,” says Dr. Harding, a former Reds team physician. “I always couch the term very, very carefully when I'm talking about it because it offends children very much — little boys and little girls. I don't say "you throw like a girl.' ”
http://www.enquirer.com/editions/1999/04/14/loc_it_hurt... (he's got the specifics on his website - those of you that can't throw a ball properly should try this as homework!)