There's no way to know/remember where I finished reading a thread.
If I'm reading a regular forum it's easy enough to remember I was on Page 3 of a topic, and when I return and see it's on Page 5, I can click the "3" to get (roughly) back to where I was and continue to the end.
I can't do that on Discourse, and it's fancy "remember where you stopped reading" functionality doesn't work for non logged in people. I tend to have to hit "End" to go to the bottom and then keep paging up til I can find my place. For some of the 400 to 500 post threads here, that's quite difficult and annoying.
From reading the complaints and responding posts from @sam and @codinghorror I would guess that they don't really care too much about lurkers as Discourse is all about improving how people communicate (ie the logged in users) and lurkers aren't really a priority... if so, fair enough. I can log in to lurk I guess.
But for what it's worth, I think most forums have lurkers and it might be an idea to support them somehow...