Ranked in these QuestionsQuestion Ranking
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 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 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.