Some people are probably pretty nervous about reviewing a comic, and aren't sure where to start, so I figure we should share some tips on how to write a decent review. It's not nearly as hard as you might think, and learning this may help you in school and stuff too!

Step 1: READ
Sometimes this is a chore. I've sat and read through a few hundred pages of something I hate or think is boring in the name of writing a review before. But if I haven't read the whole thing, I can't give the best critique I can. To write on something, ideally, you must know everything that the writer has put up. You can't complain a comic is consistently boring if you didn't read the whole thing, 'cause maybe you missed a really awesome storyarc! So be careful to read everything.

Step 2: THINK BIG
The first thing I do is to decide what my big, overall view of the comic was. If I had to sum it up into a sentence or two, what would I say about this comic?

Step 3: THINK SMALL
Now you break things down into sections which you concentrate on more closely. Personally I like to use simple, big categories like 'plot+dialogue', 'art' and 'other', but some reviews are much more organized, encompassing five or six categories, with distinctions like 'layout' and 'colour' made as separate things.
Think specifically about each thing.

Here are some things to think about when you're writing a review. You don't have to answer all these questions, just think over which seem relevant to the comic you're looking at. Obviously for a gag strip, the story may be irrelevant. Just be smart and think about the comic you're writing about:

Dialogue: Was it easy to read? Were the spelling and grammar good or bad? Did the characters speak distinctively from each other? Was it interesting? Was it witty? Did you laugh at the jokes? Could you understand what was going on?
Story: Did the story capture your imagination? Was it original or was it very cliched? Did it move too fast or too slow? Was it confusing?
Tone: What was the feel of the comic? Was it dark and gritty? Bright and Funny? Was the tone consistant? Did the tone of the dialogue, story and artwork match each other well? Did they mis-match? Did they contrast in an interesting way?
Drawing: Did the artist's style make it easy to follow the plot? Was it technically well drawn? Was there effort put in? Was the anatomy of the characters good? Was it consistant? Were the characters expressive? Did the backgrounds look good? Was it original in style? DID YOU LIKE IT?
Colour: Did the colouring work in harmony with the drawing? Did it fit the tone of the comic? Did it enhance the mood? If Black and White- was the toning good? Did it fit the mood? Were the pages balaced shadow and light-wise?
Layout: Could you follow the panels easily or did you get lost over which panel came after which? Were the layouts interesting? Did it read in Japanese left-to-right format, and was that ok? Were the pages balanced? Was timing used well?

Step 4: SUM UP
At the end, it's best to write a little sum-up to end your review. A few nice punchy sentances. If there were any bits you felt could be improved on, say how you think these could be improved, ie. 'I felt that the pace of this comic was a little slow, and that it would be more exciting to read if the creator cut out some unessesary dialogue' or 'I thought that the text was hard to read handwritten, and that a trip to dafont.com or blambot would really benefit this comic.' ALSO, if you're worried about offending the writer 'cause you've been harsh, this is also a reaaally good place to put a 'but that's just my opinion' style disclaimer!

Final random tips:
Try to write legibly. Nobody wants the insult of having their comic reviewed in l33t. If you write well, people will be more open to your words because you'll sound smart and professional. Expanding your vocabulary will help you find the words you need when it comes down to trying to describe something very specific. If you put effort into your reviews, it spurs on the artist to put effort into their comic! ...er...okay, that's just what I like to believe.
Try not to be abusive. You're supposed to be giving a balanced, critical opinion of a piece of modern (or even post-modern) literature. 'This comic sucks!' isn't very helpful.
On the other hand...Don't be banal. Faint praise isn't useful to anybody apart from for a minor ego boost. 'This comic is quite good' irks me far more than 'I think your comic is confusing', because 'quite good' has absolutely nothing which helps me and no sign of independant thought or critical judgement. Don't be afraid to outright say that you liked something, or even absolutely loved something, so long as you try to say exactly WHY you liked it.

I hope that helps some of you. Happy Reviewing!

...Feel free to add your own tips and ideas to this, or to ask questions.

Some people like to use a less formal tone for reviews, in order to avoid sounding snobby. However, a formal tone can make you sound more polite, and an informal tone can make you sound flippant or even arrogant. Sounding like a snob will only happen if you attempt to make a real effort to drop terms and names that you don't expect other people to know, or if you constantly belittle them. Obviously, many people unintentionally sound snobby as non-sprite artists reviewing sprite comics, 'of course, this is just a sprite comic, and the creator is clearly unaware of the works of Scott McCloud...', saying stuff like that would make you sound pretty snobby. Another obvious way to sound snobby, linked to this, is to say that you refused to read it, or had your enjoyment killed just because it was manga/non-manga styled, against your preference, which is very immature in a review.

Another trend among reviews on this forum that I dislike is people writing reviews adressing the creator, so that all the sentances are like 'you seem quite good at colouring' etc. Now, so far as I know, you are writing a review, and it's called a REVIEW, not a critique. If I asked for a 'critique', then I would expect the advice to be aimed at me personally, but a review...well, think about news papers. Have you ever seen, in a news paper, 'George Lucas: Star Wars Episode I started out looking really good, but then you introduced this irritating comic character and ruined the whole thing. I thought your special effects were pretty impressive though.', No, you don't see that.
We are not writing reviews just for the creators, we're writing them for other people too. When I write a review, I write it for people who are looking for comics to read. If I'm looking though reviews of comics to find a good one to read, I want to read a nice, rounded, balanced review, not just a critique that's only aimed at the person who made the comic. A review tells people what kind of a comic it is, and gives a basic guide to whether it's worth reading, maybe throws in what could be improved, in case the creator actually happens to read. I don't automatically assume the creator of any comic I review is actually going to read my reviews.
...Of course, that's just my one P (the exchange rate suggests that the equivalent of two cents would be about one pence at the moment) but I think people whould keep in mind the difference between a review and a critique before they start writing things aimed soley at the creator, and not the readers.