Comics are a more unfamiliar medium, so it may take reading a few comic reviews to fully 'get it'. However, because of the large similarities that most comic books share, reviewing them is considerably easier and less vague or confusing.
Unlike movies, I won't be splitting comics up into categories since, as said earlier, most comics tend to be quite similar. I will denote whether I read a particular comic as a graphic novel (which are either really long comics or a whole bunch of issues collected together as a single title) or a regular comic issue (which are normally about 25 pages in length).
I will also review them based on multiple criteria; 3, to be exact. They are:
Writing and Dialogue: Technically two criteria, which I may split apart when reviewing graphic novels, but given the small size of regular comics I think it's a solid criteria. Basically, I'll judge both the direction and plot of the comic, as well as the writing itself (think dialogue). This will almost always be the most important factor in a review, though failing in the other criterion will not be excused.
Art: They're comic books, not novels, so the art obviously has to be an important factor. While I don't consider it as important as the writing itself, I think comics should still have good, original art directions and believable character and environment designs.
Fan service (for lack of a better term...): There's a reason comics have hundreds of issues that are all bought and loved by its fans. Readers don't read comics exclusively for the stories (novels are usually superior), nor for the art (uh...I guess the internet takes this one). It's important to note why DC and Marvel are the two comic companies that most often come to mind. It's not because they make the best stories (far from it), but because of they have comic books based on the characters that readers all know and love. That is what this criterion will be judging: whether a comic has enough fanservice in it to make it worthwhile. For example, does a Batman comic portray Batman properly, and have enough well-known Bat content (villains, supporting characters, references to known storylines) to make it an exciting read for Batman fans? Obviously, this criterion isn't always relevant, and isn't as important as the other two categories, but I think it's still an important factor when judging comics.
I'll leave it that for now, since reading a few comic reviews will be more helpful. Just two more posts and we should be getting into the reviews proper.