I do think the issue is definitely more of an adult one than a child one, Tatty. I have a sibling and so struggle to view being an only without it being through the lens of something 'missing'. DH is an only, so sees it as totally normal and has no hang ups whatsoever. DD certainly hasn't ever commented that she's lonely or in anyway unhappy with having no siblings - I don't think she's even really comprehended that it's an option, it's just her normal.
Another couple for the list of positives:
Sharing - DD is far better at sharing, and has been at every stage so far, than many of her contemporaries with siblings. Because she doesn't have to share all her things, all the time, she's much happier doing it willingly.
Language/emotional skills - Again, DD is ahead with both these things, and always has been. I'm not sure if it's an only thing, or just her personality, but I suspect there's a link between the two and because her language is probably ahead as a result of spending most of her time conversing with adults, it has has a knock on effect on her ability to verbalise and identify emotions both in herself and others.
Like Boys says, being able to entertain themselves is a definite positive. It has huge benefits for the adults when the child is little and the child themselves as they grow up. I regularly come away from play dates with friends who have several kids and wonder how they don't go stir crazy with the constant demands for attention and general sibling fighting. It's one of the times I'm actually smug that we have an only! 