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 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.
Pro Expert-level questions & answers
Many topics have active members who are among the top professionals in their fields. MathOverflow, for example, has Fields Medal winners.
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.
Con Objective; Not Subjective
The Stack Exchange sites are specifically designed to work for objective questions; questions with subjective answers are explicitly discouraged.
See their blog for more information.
Con Limited in scope
There is a large number of different "stackexchanges", but they are all completely closed off from one another.
Con Intolerant and immature culture of the community
The people on the platform aren't very outgoing or accepting of others.
Con Popularity contest feeling
The reward system feels like you're in a popularity contest. While this encourages good answers, it feels like everything is built around the reward system. So much that, you need to have certain points to unlock features on the site.