I'm often annoyed at questions that Wikipedia answers comprehensively and with quotable references.
On the one hand, asking such a question shows a lack of initial research. On programming Q&A sites it is common practice to not answer questions that lack a snippet of code showing an attempt to solve the problem. On this site, using Google to search for an easily available answer is (or should be) a prerequisite to asking other members to do time consuming research.
On the other hand, why invent the wheel a second time? Following Jeromy Anglim's motivation that with our work here we want to make the web a better place by providing information, I don't see ourselves in competition with Wikipedia and similar public and commonly known resources. If those sites already provide a good answer, then our goal is met and we need not outrival them.