Something that bothers me about dA is the comments. Always, always whenever someone does a good job on a piece, everyone just fawns over them and is like "oh, I like this" or "this is really cool!!! ". I try not to comment very much and even more rarely leave a comment along those lines. I usually breeze through the comments a few times, looking at what people write and it is usually something like stated above. The artist most likely won't respond to that at all, or if they do, it's with a "Thank you", a "ty" or "Thanks!" These are all unbiased responses, which is fine, especially when you get about 300+ comments on a piece all stating the exact same thing. *coughs lightly*
I've found, however, that most people who respond asking a few questions or pointing out things that they have noticed (ex. choice of colour, pose, tone of the piece) usually get much more of a response from the artist. These are the types of comments that I try to leave, commenting on the actual piece, as well as a compliment and statement of my like for it. I usually ask a question about it and these sometimes go unnoticed but MUCH more often do people respond and sometimes small discussions involving the piece evolve. Most people don't read the comments of other artists' pieces, so it isn't unfair to those if you take a bit of the artists' time to discuss the piece where you posted a comment.
Actual interest in the piece other than an "Oh, this is really cool" and a favourite added to your account goes a lot farther than a quick statement of liking the piece. I like to be challenged, I like it when people tell me "Oh, this shouldn't be like this, you should do this and this to get the effect you were looking for". I like when people criticize my works and I like it even more when these people know what they're talking about. Sure, it's nice to get a compliment, but it strikes me as being more complimentary when someone tries to correct something I did because it makes me feel like they want me to do better. I try to give a bit of my own knowledge with every piece. I took four years of art in high school and studied a lot of different styles. I want to take some art history classes, too, in college as well as some additional attempts to increase my talent. Getting side-tracked, though...

All in all, I think people should leave more intelligent comments on artworks that they genuinely like.