Tumblr is...the hipster's blogger. It's that big fad that only a privileged few know how to use "properly." I had to dig on Google to get answers to some incredibly basic user interface questions. Which really, REALLY irked me. Because honestly, such a well known site should be user-friendly enough to at least point you toward an FAQ page right off the bat instead of dumping you right in the middle of the desert and making you fend for yourself.
The post interface is pretty straight forward. But the rest? Not so much. Took me a while to figure out how the eff to reply to someone's post. I still find it odd that it's impossible to reply to a reply. And the Disqus feature is useful, yes, but I kind of had to stumble upon that as well.