Here are a few techniques, tips, and tricks I have for anyone who's interested in writing webcomic reviews.
1. Write about what interests you. Writing reviews should be fun, so reviewers should write them in whatever way's enjoyable to them. Personally, I don't like writing about good artwork, and on several occasions I've limited my art section to just a few sentences rather than forcing myself to try to put words on the screen. Other times, I've written a lot about the artwork, while keep my writing section relatively brief since I didn't have much to say about that area. If you're particularly into a certain element, like setting or lettering, then go ahead and write more about it. One technique I often use is to jump between the different sections and write about the most interesting subjects first, as it's easier for me to keep writing once I've already got started. For example, I usually write the website section last, even though it's the first section readers see.
2. Find an angle. A lot of webcomics are fairly similar, which means they have similar strengths and weaknesses. No one wants to read or write the same thing over and over, so when I'm reading a webcomic, I try to hone in on a particularly unique aspect of that webcomic, preferably something I haven't written about previously. It could be an awkward scene, an oddball character, a creative technique, or a social taboo. I find it more enjoyable to focus on a novel topic like this than writing about something like a lack of character development, which is a fault most webcomics have. Sexual subjects are a particularly good angle, as they make a big impression on readers.
3. Do some research. Information about the creator and the projects they've worked on may be relevant to your review. You might choose to look at articles, interviews, and reviews about the comic for some material. Other useful sources of information are author and reader comments. In more than one review, I've quoted readers' complaints instead of writing my own, which I think helps the review come across as more objective.
Another bit of information that can be important is the creator's age. Not every webcartoonist displays their age on their site, but many have a DeviantArt or Smack Jeeves account, which often display personal information. If I see that a creator's still in high school, I'll try to be more lenient and encouraging, whereas if I know the creator's older than 30, then I think it's reasonable to have higher expectations. The creator's location can also be relevant at times, especially if their English is weak.
4. Don't use exclamation marks. This may seem like a strange suggestion, as exclamation marks are used commonly, but they add nothing to a review while making the reviewer seem unreliable. Reviewers sometimes use them with their positive comments to offset the review's negativity, but this is a shallow technique that could be replaced with careful word selection. Overly enthusiastic phrases, such as "The coloring looks fantastic!", actually undermine the reviewer's credibility by displaying their inability to convey their opinions in a more mature fashion. A sufficient explanation of why something's good or bad speaks for itself.
5. Pad subjective criticism with objective criticism. It takes a bit of time to establish credibility and ease readers into a review, so I prefer to start the review off by analyzing the comic's website, which is the most objective and impersonal part of a webcomic. Website issues tend to be pretty obvious, and I've yet to see someone get emotionally upset because someone criticized their site as being bland and/or not entirely functional. After getting the audience to agree with your obvious statements, the most subjective part, the writing section, becomes more palatable. Even there, it makes sense to continue to ease the reader in by introducing the writing with more obvious elements, like the genre, the story's setting, and brief descriptions of the main characters. My writing sections are generally on the negative side, so it's important for me to follow it up with the art section, which is both more objective and, usually, more positive, as webcomics tend to have better art than writing. I also often try to end a review on a positive note, focusing on ways the webcomic could be improved.
6. Mix it up. When choosing the next webcomic to review, I often factor in the results of my last few reviews. If I just did a few positive reviews, then I look for a webcomic that I expect I won't like, and if I've been doing a lot of negative reviews, then I look for something I think I'd enjoy. These "mix it up" reviews are probably the most fun to write, as many times my expectations have ended up being way off.
7. Combine micro-criticism with macro-criticism. In webcomic terms, this means combining criticism of individual pages or strips with analysis of the webcomic as a whole. Many reviews are overly brief, and it's largely due to a deficiency in one of these areas. The most obvious example of doing this correctly is when a reviewer backs up a general statement about the webcomic by linking to several pages or strips where the aspect in question's particularly apparent. It's also possible for reviewers to successfully prioritize one over the other, though. For instance, micro-criticism can be focused on by doing panel-by-panel breakdowns of particularly noteworthy strips, while macro-criticism can be focused on by generally comparing the webcomic to others in its genre, or to point out a particular trend in webcomics.
8. Buy The Associated Press Stylebook. It might seem a little excessive to buy a book just for writing webcomic reviews, but from what I've seen, most reviewers regularly do other writing as well, such as creative writing or blogging. I've personally found it very useful to have such a quick and easy reference for all my style questions. A brand-new, 2012 edition of the stylebook would probably cost $20 or more, but the price drops significantly if you get an older edition. The stylebook I own, which is the 2007 edition, can be bought new on Amazon.com for $3.50. I use a combination of AP and MLA styles, so I don't adhere to the stylebook completely, but I still refer to it fairly often.
9. Keep your tone consistent. I always proofread my reviews before I post them, and often I find that in some parts I'm being a jerk, while in other parts I'm being too nice. Perceptions are unstable, and it's not unusual that I might initially have a very negative perspective of a webcomic, but then I start to realize some things the webcomic does well while I'm writing the review. After all, it can be kind of a chore getting through 50-plus pages of a webcomic I don't like, so it makes sense that I might be sarcastic or condescending when I start writing about it. Mood, health, concentration, time constraints, and tiredness can also have an effect on how people write. Always give your writing a second look before you post it, as changing the way you word some of your criticisms can prevent an unintentional insult.
10. Think like a reader. Readers are as judgmental as critics are; they just tend to keep their opinions to themselves. Even if a reader can barely draw a stick figure and hasn't even penned a short story, they evaluate every comic they see to determine if it's worth their time to keep reading it. Many reviewers feel timid about criticizing webcomics, but readers do it anonymously all the time. So, think like a reader: Did you enjoy reading this comic? Would you consider reading it regularly? Would you send your friends a link to it? If a webcomic isn't appealing to you, then there are probably a lot of readers out there who don't read it for the same reasons.