Another only here. I actually agree with JdotJ about social skills, though it may be down to personality as much as being an only child. I did have friends, but went to an all girls school, and felt very awkward in mixed company for a few years after I left school.
I didn't like being an only child, partly because there was so much focus on my behaviour so that I wasn't "spoilt". I had to be a "good" girl and never got away with anything!
I ended up with 5 children - certainly didn't want just 1. When they were young I realised what I had missed out on - squabbling and fighting as well as fun times together.