There are not a lot of them that I have found I like as a user. Basically, they need to allow the user to authenticate with their service via 3rd party accounts (Google, Facebook, Twitter, etc.). With no further requirements. The comment service should be nearly invisible to the user. It is just a means to an end -- to get comments on your post.
I used to participate in Disqus comments, but now I don't like it and will generally not leave a comment before I use it. Because they switched to prompting the me to fill in Disqus account information before allowing me to comment. I don't want or care about managing a Disqus account... I only want to leave comments. And Disqus also doesn't work well if you turn off 3rd party cookies, which I disable in every browser I use.
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