Karp’s thinking about the comments section, which is generally assumed to be a core blog feature, helps illustrate his broader ideas about how design shapes behavior online. Typically, a YouTube video or blog post or article on a newspaper’s site is the dominant object, with comments strewed below it, buried like so much garbage. Thus many commenters feel they must scream to be noticed, and do so in all caps, profanely and with maximum hyperbole. This, Karp argues, brings out the worst in people, so Tumblr’s design does not include a comments section.
How, then, to encourage feedback while discouraging drive-by hecklers who make you never want to post again? First, Karp notes, you can comment on someone else’s post, by reblogging it and adding your reaction. But that reaction appears on your Tumblr, not the one you’re commenting on. “So if you’re going to be a jerk, you’re looking like a jerk in your own space, and my space is still pristine,” Karp explains. This makes for a thoughtful network and encourages expression and, ultimately, creativity. “That’s how you can design to make a community more positive.”
Another example: when users wanted a private message system, Tumblr’s answer involved clicking on the words “Send Fan Mail” — a prompt to encourage positivity. “We don’t want to allow you to have your feelings hurt on Tumblr,” Zack Sultan, a Tumblr designer, says.
Or you could just leave a comment in the blog’s comment section:

I don’t understand why Karp keeps saying Tumblr doesn’t have comments when it clearly does. It might have a different name and be a slightly better implementation but it is comments. Your “replies” even show up under the person’s post on their blog (optionally), just like comments! Have you ever heard the phrase “lipstick on a pig”, David?
This post's short URL is http://tmblr.co/Z8qtbyPHz4U0
You have been reading a blog.