I've said most of this before, but this certainly is the best place to say it again.

You should not feel guilty for not commenting if you don't have the time or can't think of anything useful or interesting to say.  If you have nothing to say, I see no reason for you to feel obligated to say anything at all.  Also, the reciprocal system that's already been mentioned I think actually is hurting the community -- it's becoming more and more "uncool" to offer genuine advice and put real thought into your comments.  I don't want anyone commenting on my stuff because they feel like they owe me something.  Respect is infinitely more satisfying than guilt.

However, in terms of commenting in general, I think it is a tad selfish to just post images and never comment on anyone else's.  It makes you look like a snobbish, "too cool for this website" type.  If you have some artistic knowledge to share, by all means, DO IT.  You never know who might benefit from your input and encouragement.

I've only had one experience with somebody sending me IMs that "gently hinted" that I should drop by and leave a comment or two.  I'm not sure if it was his intention to be annoying or manipulative because he clearly wanted advice, as opposed to simple attention, but still I think that's rather self-centered behavior.  If I'm interested in your work, I'll check up on you.

The "copy and paste" comments are, I think, very disrespectful to the artist.  It's pretty much saying "I'm too good to say anything thoughtful but I still want people to think I'm nice".  That's not to say that everyone who does that is a jerk; they could just want to show their respect but not have the time to be eloquent, or they could just not have been here long enough to understand how common and irritating micro-comments are (I made that mistake a few times in my first month or so here, and I apologize -- I thought the word "woah" over and over again was the ultimate compliment).

Also, I think constructive criticism, when worded carefully and politely, is the best thing in the world for an artist.  I even appreciate the constructive crit I've gotten that I didn't agree with, because it's the thought that counts.  The thought that someone actually cared enough to tell me how they think my image could be improved.  Alot of people don't have the spine to accept true, helpful comments for what they are, and if you run into one of them, just back away slowly like you would from a rabid dog.  You've shown them respect by offering them advice, and if they can't take it, they can shove it.

And lastly, I want to say this directly to Zacko: you don't have to comment in my gallery anymore, even if I leave comments in yours.  Trying to be a Comment Wizard is clearly causing you stress and I figure one less gallery to worry about might do you some good.  I really respect you, and although I am honored by your critique, if it is a chore to offer it than you probably shouldn't.  Thanks for being awesome.