I strongly disagree with your saying that reviewing can take as little time as 'only a few minutes'. Reviewing takes a long time; you have to factor in listenning to the song
several times to pull out as many aspects out of it as you can to talk about. It's important to thorough if you are evaluating something, especially for music. It also takes a long time to organize your thoughts, and then another good while to actually write down everything you want to talk about into a coherent presentation that people will find interesting to read. Reviewing is a task that you have to commit yourself to.
But anyway, yeah, a set of active reviewers looks good for the site, so get out there and start reviewing!
As for me, I'm essentially using the lack-of-time/too-busy excuse that everybody hates to account for my not having reviewed in years. (my later reviews weren't very well written anyway, in my opinion.) I do try to comment on tunes every now and then with the commenting system. That, I can agree that it can be done in a few minutes.
EDIT:
@Jojo: Talk to m0d about joining the reviewing crew. I'm not sure if there is a form up to apply to be a reviewer yet (you could look around for it I guess). It was one of the last things on the list of stuff to take care of when the renewal of the site, if I remember correctly, so there may not be anything there yet. Usually reviewer applicants were required to write a 'test review' assigned by an admin.