Until recently, Tumblr didn’t support comments (natively). They’ve implemented them as what they call “replies”. I think there has to be a question in your post for you to enable replies (at least, that’s what makes the “Allow Replies” checkbox appear). Even then, replies are limited to 250 characters.
Now, I believe that if you’re going to enable comments, go full-force. If you’re allowed to write a full post, and you ask for my reply, why can’t I give a full reply? Either stand on your soapbox and leave it as that, or let me stand next to you for a moment so I can state my opinion when you’re done.
What can I do with 250 characters? Should I write my own tumble and link to it? Should I just repost your article and comment on that? There’s not a lot you can fit in that kind of limit and give a well thought-out response.
Tumblr: let me say a little more, please. I promise, it will be well thought out.