Ranked in these QuestionsQuestion Ranking
Pro Popularity / maturity
The Stack Exchange sites have a large user base.
Due to this being broken down to sub-sites per topic however some of their sites have more users than others, so depending on the subject matter of the question this pro may not apply.
Pro User ratings by topic
Users are scored based on their contributions, with the context of those contributions also being scored (i.e. answering a question tagged as being about XML increases that user's XML score). This results in allowing the questioner to gauge a rough idea of the answerer's relative knowledge within a domain when considering their answers vs contradictory answers from other users.
Pro Subject Matter: Various (site specific)
Whilst sites under the Stack Exchange banner are focussed on specific topics, there are a wide variety of such sites.
As such Stack Exchange could be described as being open to questions on any subject matter, but has the advantage that within its sub-sites, questions are targeted to the specific users/audiences of those sites.