Last week I received a request to review a manuscript for a journal, and for the first time, I had to think for a few days whether to accept it or not. Not that I receive requests every day, but I've had my share.
I've only declined an invitation twice. The first one, because it was an article on, let's say, a Costa Rica author from the 1940s (not real decade nor country, but the analogy applies very well). I don't think I ever read a book by a writer of such nationality, let alone from the 1940s, so I was obviously unqualified to be a reviewer. The second time I declined, it was an article on Borges. Just because I'm from Argentina and do research on Argentine literature doesn't mean I can judge the merits of an article on Borges. Furthermore, there are plenty of Borges specialists in academia, and the request came from a well-known journal, so it would not be hard for them to find a better match.
The reason why I hesitated this time was that, though it wasn't a topic I specialize in, I have read and thought about some of the issues the article (at least judging from its title and abstract) raises. And I am interested in them. So I finally accepted.
So my question for you, academic readers, is: provided there are no other issues (lack of time, etc), how far outside of your comfort zone are you willing to go when reviewing a manuscript for a journal?