I was just having a look at github's new tree slider.
In any case, it got me interested in the importance of transitions. Back in the old, pre-ajax, pre-broadband days, we would click on a link, go get a cup of coffee and then come back to a new screen. In the meantime the little "loading" icon would spin which would let us know that something was happening.
Nowadays, not only is it possible to change part of a page without reloading the whole page (thanks to AJAX), but with our new browsers and broadband connections, this could change so fast that we don't even notice.
Now however, they seem a bit more of a necessity. They let the user know what happened on the page and where.
Going forward, it would be nice to see some conventions appear as well. Fade in for dialogs, slide left for new page, slide right for old, etc...