The lack of comment threads is a feature. It prevents other people from interjecting into the story you want to tell with your blog. For example, if you post so

The lack of comment threads is a feature. It prevents other people from interjecting into the story you want to tell with your blog. For example, if you post something on YouTube it's kind of inevitable that someone will come along and comment that they hate your face, whereas on Tumblr you can write that comment but you have to reblog the item to your own blog to do it. Also, your reblogs are pushed to your followers' streams but people viewing the original post don't see it, so if you write something nasty it just makes you look bad.