Here are a list of impediments I come across regularly with commenting systems on blogging platforms. This is a personal list, so obviously you'll see my likes and dislikes here. I am not targetting any particular blogger, so don't take it personally.

For what it is worth, here are my pet annoyances.

1. CAPTCHAs - I've said enough about this in the past, so I won't elaborate except saying that typing down civjsiwep or 53912681 every time I want to comment on something is no fun. It gets worse over time, not better. Asking me what is 10+2 is slightly better, but still a nuisance.
2. Moderated comments - while not annoying as much, author-moderated comments (where comments appear only after approval) break the flow of conversation since all traffic is directed through one person. It really slows down discussion.
3. Comment popup windows - definitely most annoying. I prefer inline commenting systems. Popup comment forms are awkward.
4. Wrong tab order in comment form - Blogger.com is the main culprit here. You would think google would have fixed something as basic as this after so much fanfare about the "new blogger". But they haven't. The tab order on the commenting fields should make it easy for me to post comments without using the mouse. Every time I use the mouse to highlight a field, I get annoyed. I'm sure a lot of bloggers feel the same.
5. Not "remembering" comment authors - I like the system where a cookie is stored on the end-user's machine and once a name/e-mail/URL is submitted, I don't have to type it every time before commenting. Of course, I can always remove that cookie from the browser, so that's not a problem. Having to fill out name/e-mail/URL for every new comment is definitely a chore though.
6. Aggressive comment spam filters - I don't want to be told that my IP is blacklisted and I should get a "anti-spyware tool" because my computer may be infected. Duh, I use Linux. It's not my problem that anti-spam authors have no idea about dynamic and shared IPs used by public ISPs in many countries and the stupidity of implementing IP blacklists in the first place.
7. No acknowledgment - The best incentive to keep commenting is acknowledgment from the author. If I don't get acknowledgment over a period of time, my motivation to continue commenting is definitely reduced. Even if it's something as simple as "thank you" I prefer that to nothing, especially if I've posted a long, thoughtful comment
8. No reciprocity - I generally do not expect anybody to post a comment on my blog just because I posted one on theirs. However, over time, I do expect some form of "giving back." Either through links or through the occasional reciprocal comment. Nobody likes being neglected.
9. Too cluttered - If the blog style is too cluttered, I have to search for ten minutes before I can find a small "comment" link tucked away somewhere in the corner.
10. Requiring registration for commenting - Blogs are not discussion forums. So mandatory registration before posting is definitely a way of saying "no comments invited here." There are better ways to fight spam.

I hope I've not annoyed too many of you out there. However, if you are guilty of any of the above and I still comment on your website, it's because I really like your content enough to put up with all that. So be flattered.