I really do not look too often, but have you found the same commenters making statements that suggest they are using double standards?
I cannot speak for anyone else, but issues of personal preference are often so complicated that while they may seem contrary on the surface, they actually could be two different viewpoints.
Some commenters also blur the distinction between the actor/character/previous character played. I think this one point probably covers a lot of the instances of double standards.
I have also seen more comments that seem to judge actresses more harshly than their male counterparts. If a majority of the MDL commenters are female (which I have been led to believe), it might also help to explain some of the discrepancy between comments about actors and actresses. There are also comments that seem to suggest that only certain actresses match a certain actor as a couple in a drama. Maybe those who do not have a problem simply do not comment, so it just seems that way. I think that dramas, especially ones that feature romance, are more about chemistry and talent than beauty.
A few unpleasant commenters can ruin a good drama comment section. Some people might also not be able to express themselves in English as well as they would like. I can also say firsthand that some dramas draw you into their world and make you want to defend a story, action, character, or couple. Commenters might feel the same way about an actor or actress as well.
Overall, I find the comment section both enjoyable and frustrating. It is also worth the effort when you want to discuss Asian dramas, but people around you have no interest (a lot of the world outside of Asia).