Interestingly, I find that a lot of the questions on Stack Overflow are from such noobs that I'd have to spend time explaining how to program before getting to their actual question. This disuades me from participating, as it's hard to find the questions from programmers who I could help.

As a professional, if I can't find a way to contribute my expertise in a timely and efficient way, then I'll get on with something interesting and more lucrative. Which is a shame, because there is a lot of useful stuff on SO, and I'd like to contribute at a high level, but I'm not willing to put in 2 hours a day to find the 1 or 2 questions that would be worth the time.

I'm an older programmer, and this may be one explanation of why older programmers answers on SO aren't as useful - the good ones are not the ones answering questions!

This is just an anecdote, but one that certainly applies to me