From an "outsider's" perspective (as in, I am neither a parent nor an only child) the one thing I noticed, growing up close friends with a number of single kids, is that the single kids were often socially on-par with the homeschooled kids I know.
As in, they are wonderful, mature, intelligent, people who could easily relate to adults but they had a much harder time relating to people their own age in a normal way (either much too clingy or much too "who needs other people - leave me alone, you're too 'immature' for MY mind"). That's not to say they could never relate easily but most of the singles I know that achieved that were either super extroverted from the beginning or made a conscience decision to learn how to socialize with their peers.
Of course, I was always one of the "odd" ones and also had to make the decision to figure out how to interact with my peers in a way they found less "strange" (I grew up very close with my brother - basically like twins that happened to come out a year apart and our parents have always talked to us like we're grown-ups, even before we could talk so we showed up for kindergarten with huge vocabularies, talking the way adults do, etc - made for an interesting time, haha) but I was able to do it, unlike some of the singles I know.
There's nothing wrong with being different but I've always felt sympathetic to the super introverted kids who grew up without a sibling, spending a lot of time alone from a young age, etc. Maybe it's one of those "don't know what your missing" things but I know as a VERY introverted person who grew up rather socially (my mom knew I was going to be really introverted from the time I was very little so she made an effort to get me out there so I wouldn't end up more inside myself than I was already going to be), it was darn hard for me to assimilate into life. As it is, I'm only now, nearly done with college, coming into my own where all my friends appear to see me as "the cool quirky one"...and of course, that makes no sense to me but annnnyway.
Basically, my bottom line: socialization with a variety of peers!
On the other hand, sometimes I wish I had been an only child! None of this 'brother getting away with stuff because he can' junk!