The way your comment system works is part of what defines your site, and if that part is not under your control, you'll be just an average website with an average community.
Think of all the VB/phpBB/IPB powered forums.
Treating commenting systems like a commodity scream "I'm an average website", or "I'm not a technology site".
For the same reason, I never use a CMS. If I want to build a website, I will build it from scratch. CMS powered sites tend to all look a like, you can often tell right away when a site is using a certain CMS; by the lack of polish in its design and the familiar menu to the left, or the familiar urls, or the familiar clutter and noise that always accompany sites powered by X (where X is some CMS).
Tumblr does a good job of providing high quality designs, so your tumblr blog will always look polished and professional. But it will also look like the other thousand blogs that use the same theme you're using. You can make your own theme, but if you're a control freak, you'll soon run into walls where there are certain things you want to do with your blog but you just can't.