I really do dig Mountain Lion's Notification Center. Every once in a while, though, an alert pops up that gets in the way of me clicking something underneath it. Or, you know, one appears telling me that my grandpa has sent me an offensive joke email, and a client happens to be looking over my shoulder at that exact moment.
Whatever the case, you can dismiss those pop-ups if they're in your way pretty darned easily. If you visit System Preferences > Notifications, you'll see that they come in two different flavors for any app that'll use them—banners and alerts.
Alert-style notifications require you to click on them to make them disappear.
Banner-style ones, on the other hand, disappear by themselves after five seconds.
I much prefer the banner notifications, as I never want to have some floating window waiting for me to click on it. But there's no obvious button to make one scram if it's in your way. You've just gotta wait five seconds, right? Nope! One way to do it is to click on the notification and drag it off the screen to the right.
If you've got a multitouch device like a trackpad, you can also just swipe from left to right over the notification with two fingers. Handy!
For some notifications (software updates, for example), dismissing them in this way has the same effect as hitting a snooze button—they'll come back. But for pop-ups like email alerts, this'll get them out of your face. And out of the face of anyone else who happens to be looking at your screen, thank goodness.
This tip was originally suggested by my esteemed colleague, Jochen Wolters, who is so cool that it should be illegal.