LOL at the bogey analysis... have that yet to come
Of course, there have been some very good points made, of course there will be disadvantages - but none that can be got around as long as you are aware of them and act accordingly.
Of course, there can be socialisation issues (please don't do what my parents did and move to a village where they knew no-one then spent most of my childhood complaining if they had to ferry me anywhere - it did mean I had very few friends growing up - but that's really a parenting issue rather than an only child issue).
Of course there can be extra stress put on the child in terms of expectations - but if you're aware that is a danger, you can catch yourself and take a few steps back. I think I do feel it more keenly as an adult since my father died - I am the ONLY one responsible for my mother and I've gone through a bit of a rollercoaster on that one.
Then again on the flip side, I think I've come through with a fairly self-sufficient, confident personality, and I'm laying down the law with my mother a bit more - so those two major disadvantages (even though not addressed in my parents' parenting style) have been dealt with by me as an adult anyway.
And besides - being perfect is boring. We all need our character flaws to be interesting, well-rounded individuals