Some of you may be familiar with the OS X add-on Growl, which displays little messages that programs send in a pop-up window that does not take focus. Other programs, such as Azureus, have this ability built in without using Growl. Windows has had this feature for a long time, so why has Apple never implemented it?
There is a very sensible reason. Remember the icons in the dock that jump up and down when an application needs attention? Rather than throwing a window your way like most Windows applications would, often resulting in text or even passwords being lost or disclosed by typing them into the wrong window that has just popped up, OS X is courteous and kindly asks the user for some of his attention, "when you're ready".
Moreover, it does so in an area of the screen that is usually guaranteed not to be used for any other purpose. Growl, on the other hand, could quite conceivably pop up in an area of the screen that the user is actually performing work in, and prevent the user from executing a mouse action, and break his concentration. Some may intuit that the problem could be alleviated by having Growl messages displayed in a separate window, which would then cause its icon to bob, being more courteous. But this is actually worse than having the original application take such action, because the user couldn't tell at a glance which application was calling him. On the other hand, applications use Growl to display many more messages than they would usually display of their own. So perhaps they should not be displaying these messages at all, since doing so would simply lead to an inflation of messages, and an all-dancing desktop. If that's what we wanted, we'd be using Windows, right?