This comes from talking to a couple of people elsewhere.
Let's say you're considering whether or not to leave a comment. Leave a comment.
You may wonder if the post is too old. It isn't, leave a comment. But, you might be thinking, it was written far back in the mists of ancient history and surely saying something now will be tantamount to necromancy and start the zompocalypse. Yes, it is very old. No, it won't destroy the world. Comment.
If I were being overwhelmed by comment swarms and couldn't keep up with everything and was having to fight off the vicious comments with a can of diced potatoes my grandmother bought out of fear of Y2K, perhaps there would be reason for you to hold back and not comment. But at the moment I am not worried about the comments overtaking me and gnawing off my leg, so comment already.
Seriously, I like feedback. I do not get a lot of it. I appreciate the feedback that I do get, and I don't want to downplay that any, but if you're considering whether or not to speak up, for the love of all that is holy, speak up.
Also you should know that at least two people lost comments to Blogger being jerkish. So I recommend that when you comment either write it elsewhere and then copy and paste it over (which is how I used to do all of my commenting back in the day) or copying it right before posting just in case (which I still generally do.)
Practices such as these will also serve you well on disqus and typepad.