The mark all read button is obviously incredibly dangerous, and shouldn't be clicked by accident. The absolute last place that I would expect to find that button is next to the "new topic" button.

After seeing this and being very surprised, I clicked it, assuming that, given that it would give me a warning prompt asking me whether I wanted to perform such a dangerous action. Apparently, no. If you accidentally (or out of curiosity) click the link that is right next to the new topic button, you immediately and irreversibly mark the entire site as read. That really doesn't sound like very good UX design.

Why not move it to the user menu (that you get when you click your username), and add a confirmation message to it for good luck, so people don't end up accidentally marking everything as read on a live site. It's still fairly easy to access, but it's also hard to trigger accidentally.

The master "Mark All Read" has a confirmation button. I know as I just tried it (it is still there) and there was a huge debacle about this on the preview board to get that confirmation prompt.  

OK so it looks like I clicked the "mark forum as read", which I still feel should have a confirmation prompt.