Bottom right of each post in a thread. @mention lets you tag someone so they know you're talking to them and so they get a notification.
Another way of doing this manually, which can be helpful if say, they didn't post in the thread and you want them to see it, you do [@Their name] instead.
Just thought that I should tell you. In my opinion it's useful (How many times have I used this? Like 6?)
On another note, I keep pressing the windows button on my keyboard when I try to copy-paste something and it bugs me.