1. If Joomla cache is On, then the comments do not appear in a manner that enables Googlebot to index the comments. Precious SEO juice is lost. This is an essential requirement. I am not sure if all Jcomments users have noticed the same.
2. There are too many images that needs to be downloaded. Take for example, number of images needed to get the rounded corner effect. Downloading so many images greatly reduces speed of the website as number of calls made to the server is too high.
3. There is a link that appears which tends to be appear spammy in the eyes of Googlebot. I am not sure whether the author will get any boost because of the link, but it greatly reduces Search engine friendliness of site using Jcomments.
4. Spam control is still not very robust.
However, the publisher of the comment deserves all the appreciation for the effort and keeping it non-commercial. Appeal of the extension will enhance immensely should the author choose to fix these glaring gaps.