Nothing warrants unsolicited advice quite like having a baby. I would say a close second to actually having a baby would be the unsolicited advice that comes from trying (and struggling) to get pregnant (because obviously something teenagers manage to do in the back of the car is too complicated for some of us adults).
But still... I think it is worse when there's a baby involved. It takes a village and I guess people feel like their contribution to the village is to offer any "helpful" piece of information they can remember.
In my experience, there are two types of advice:
(1) helpful advice
(2) advice people give to make themselves feel better
Personally, I don't mind helpful advice. My policy is to listen to all of the helpful advice people shell out. After all, they have been there and I'm not so arrogant to think I have it all figured out. I don't. I have managed to keep Henry alive, but that doesn't make me an expert. I try my best to listen to all of the advice and then pick what I want to put into practice. As I learned the other day at Henry's doctor appointment, that includes information from the pediatrician too (but that's for a different post).
The second type of advice is what I would categorize as "advice people give to make themselves feel better." Whether it is to make themselves feel smarter or to justify actions they have taken in the past, this advice actually does very little to serve the person the advice is given to. For example, when someone tells an expecting mother to "sleep as much as you can now. You won't be sleeping when the baby comes." First of all, expecting mothers don't sleep well, so you couldn't follow this advice even if you wanted. Secondly, reminding someone of their impending sleep deprivation solves nothing. I think this kind of advice is just a misery loves company kind of thing.
That kind of advice is really hard for me to listen to, but I usually just try to brush it off and move on. It's important to recognize that the person giving that advice feels the need to share it for some reason and I figure I am just being a sounding board for some time of baggage they feel the need to unload.
For that reason, I personally only try to give advice when I honestly think it will benefit the person receiving the advice. Other than that, I think the most important thing to do for people is to encourage them and (most of the time) keep my mouth shut.
So since you didn't ask... that's how I handle unsolicited advice. :)