One thing I enjoy about reading blogs is when people talk not only about what they like, but why. Even if I don't share their enthusiasm for a writer/artist/character/show, the thought process behind what they see in it is interesting.
On the other side of the coin, I get irritated by people who state an opinion in one sentence, without bothering to back it up with any evidence. Not so much if it's about them liking something, more when it's an assessment of the specific quality of the work. For example, "Jim Aparo is a terrible artist," something like that.
I know it's because I see an arrogance in it, someone so certain they're right they don't think it even requires explaining, or debating. We don't need to see the work, because they're obviously right, so let's just take their word for it. Also, that certainty they're correct is a little troubling to me, indicates a lack of willingness to conceive that maybe they're wrong. Which I'm sure is a nice feeling to have. I remember it being that way for me when I was that way as a dumb teenager.
Now, I find I'm not sure about much. Even when I'm pretty sure, I know there's at least on other side, and I wonder if they're right, and I'm missing something. Frankly, people with no doubts whether they're right or not worry me. I know I'm talking about comics and entertainment here, not politics or religion. It's not really a big deal whether some person on the Internet has strident opinions about whether Adventure Time is any good. But we slip into "I'm right, you're wrong" so easily I'm always leery of people who consistently operate that way.