I spend a lot of time at Starbucks, so I see the captive network assistant window quite frequently. This is the feature introduced in Lion that shows when one initially connects to a network that has some hoop to jump through before allowing access to the Internet.
In Lion, this window would automatically dismiss after the subsequent page finished loading, but I usually manually closed the window rather than waiting and seeing ads that did not interest me. With the release of Mountain Lion, however, the window dismisses as soon as the connection is available without waiting for the page to finish loading. It’s a subtle detail but a huge improvement for anyone that deals with captive networks on a regular basis.