Being tracked by a single third party on every site that uses it comes to mind.

Disqus shows ads, unless you pay like $10/month. (& tracks your visitors, like someone else mentioned.)

Maybe you would instead like the looks of EffectiveDiscussions — it's a new commenting system I'm developing. See my other comment nearby (or, if you didn't find it, go to: https://www.effectivediscussions.org/blog-comments ).

This is UX-ish, but I regularly have login/posting issues and very slow loading times off Disqus.