The major problem with Jetpack comments is that they do not let people with self-hosted sites using Wordpress CMS and simultaneously having WP.com accounts (purely because it is required for running Jetpack plugin) to comment with their site's details. Jetpack comments recognise the e-mail address and change the site from a self-hosted one to the WP.com address, which is clearly not what is desirable.

PS: That's why if Jetpack Comments is the only option (like here) I am forced to opt for Twitter authentification. And in most cases I prefer not to comment at all.