I think there are two ways to evaluate any kind of art. One is the personal form, through your tastes, the other is technical.
I'll give my example, one of the reasons I like Zoro is that he's a more serious guy, I've talked to people that this was exactly why they didn't like the character. This is totally related to personal preferences.
The technical form is objective and not subjective, point A is analyzed in such a way that it will be done well or not. Thierry Groensteen is a reference when it comes to research in comic analysis, his book The System of Comics is highly regarded in the area. I haven't had a chance to read it yet, but it's a read I'm looking forward to.