"Could an average programmer invent quicksort, dynamic programming or information theory if transported back to the mid 20th century?"
I think many could have yes, had they been in the same academic environment as the people who invented those things. It's amazing what normal people can acheive when they are isolated from the noise of modern corporate work environments and given the resources to focus on a problem.
Yes, but that needs to be balanced by the distorted form of reputation people get even when they are polite and thoughtful due to the lack of any emotional channel in text-based web forums. Add to that the fact that in real life, people have many sides to their personalities, which are inhibited or eccentuated in different contexts; but the impression the web gives of people lacks those contexts.
Then, consider the good that anonymity has allowed for; think of all the things people have able to speak out about anonymously which would have gone unsaid.