JetPack comments is great! But it has a serious drawback. We run a site with 700,000 daily UVs - we can't let just anybody leave a comment - we want people to be properly logged in with a WordPress, Twitter or Facebook login. But when we enable the setting "Users must be registered and logged in to comment" and view a post when not logged in, we don't get a comment form, just an annoying link to the WordPress login page.
From a usability and UX point of view that is terrible. Users will never bother to comment if they have to click on something to get to a comment textarea. People expect to be able to type their message in an input immediately and log in if necessary before they submit it. They won't even notice there's a link they have to click on.
I don't understand why we have to allow anyone to comment by entering any old email address just to get the JetPack textarea and social login buttons.
Hm, just realised this is also the default WordPress behaviour. That's rubbish too!