The Advantages:
Being an only child can be either good or bad, depending on how you perceive it and how you are brought up by your parents. The advantages could be that you get the undivided love and attention of your parents. They would dote on you more and provide for you more - in terms of getting you stuff, toys, etc. Obviously, being an only child means that your parents have only you to spend the money on and not have to share it among other siblings. So, you'd get more toys than normal, more money to spend than normal, more inheritance than normal, and of course more love from your parents than normal.
As an only child, you may also be spared the complications arising out of having an overbearing, unhelpful, competitive sibling. It is no fun having a sibling like this, who hoards the attention of your parents, who in the eyes of your parents can do no wrong and is doted upon to no end, often at your expense. They may well walk off with much of the inheritance by virtue of being the doted one in the family. As an only child, you would be free of any of these complications.
As an only child, you may grow up to be more independent and able to fend for yourself better, if your parents haven't spoiled you by tending to your each and every need. Not having an older sibling to help you every step of the way may in that sense be beneficial and make you capable of looking after yourself earlier in life. Therefore, being an only child can certainly have its advantages.
The Disadvantages:
As far as disadvantages go, the "Little Emperor Syndrome" comes to mind. For those unfamiliar with the term, "Little Emperor Syndrome" refers to the Chinese situation involving parents and their single child. Of course, all of you would be aware of China's one-child policy. Little Emperor Syndrome is an unintended consequence of that policy. This is a situation in which the parents lavish their love, attention, resources on this one child of theirs, and as a result, the child becomes spoilt and, well, behaves like a "Little Emperor." This sort of excessive attention and care can prove detrimental in the long term for the child. The child gets used to having everything done, managed, taken care of - by their parents. When they have to live in the real world and face real problems, they might not be able to cope with it. They may lack self-confidence to go out in the world and get things done for themselves. They might feel lost outside of the cocoon that their parents created for them. This of course need not always be the case and can be overcome by good parenting.
Perhaps the most apparent disadvantage of being an only child is the feeling of loneliness - not having a sibling to play with regularly and to be able to share your thoughts and memories with. When you parents are no longer around, not having a sibling to talk about things with or look up to for any kind of help or support can be quite a disadvantage. Also, when you parents get older, being an only child, you would have to shoulder the responsibility of taking care of your parents on your own, which might be overwhelming.
As an only child, you may also face an immense pressure put on you by your parents; for example, to keep the family name going or to do well in your academics. As an only child, you would also be watched with an eagle eye by your parents and this may be quite suffocating and stressful for you. If you had siblings, the pressure wouldn't be as much and you may perhaps be able to lead a more stress-free life. This again would be a highly subjective experience. There may be many who may feel no stress at all and are able to take this pressure, real or perceived, within their stride.
