1. People don't search properly before posting
2. There are a lot of "low quality" questions that could be answered by reading documentation (also see 1)
3. Most of the questions that apply to 1 & 2 have already been answered on StackOverflow.
4. It's overwhelmingly (US) English, so people who are not fluent pendants are down-voted.
5. The people who need the most help are those who are least suited to post, as they rarely give enough detail ([example][1] - where to start?), ask very generic questions ([how to build a website][2]), or ask questions that have been answered before
When StackOverflow started, I spent a great deal of time on it as it felt like a good way to help and share knowledge (or just show off). Nowadays, I'll vote when I find an answer (usually via Duck Duck Go), but [the last question I asked][3] is still unanswered (allowing Facebook to use this as a support forum is a joke).
I feel that it's become a victim of its own success, and is getting to the point where it's as bad as the sites it tried to replace.
[1] http://stackoverflow.com/questions/9395670/dataset-getting-m...
[2] http://stackoverflow.com/questions/87305/how-to-build-a-webs...
[3] http://stackoverflow.com/questions/8077747/what-are-facebook...