There’re several things in Growl notification display behavior that I think might be improved.
First of all, it’s a new “close” button implemented in version 1.1. It only appears when you hover a notification and lets you close the message you see no need to react on. Not even mentioning a degraded look of notifications (tastes differ) the button has a serious disadvantage as it requires quite precise positioning that eliminates the advantage. So what? Easily: just recall the Fitt’s law and let one click a larger area like the whole message… By assigning the right click to this action you let the thing be done without extra complexities. Is “right-click to close” an obvious action? No. Just like any idiom it has to be learned, but once learned it may become very handy. Arguments for are that some applications like QIP (Windows) or Google Notifier (OS X) already use the same approach successfully and both have very large user bases.
The second thing is the way Growl displays notification pool. Right now the mechanism is quite primitive, so when there’re just too many messages they may take all the screen space available thus interrupting you from work. Having a dedicated area for displays might improve situation a lot. I.e. let’s say messages can only take a single row with a maximum height of 50% of screen height. Now you may be sure that no matter what happens messages would not cover your workspace if you have it outside a suggested area. Besides, now we know that if notifications come from top-right they bottommost message is always the latest one.
Once again, this approach is already adopted by other applications.
The third improvement would work great with the second one. As we have limited our screen space, let’s make notifications use space more effectively. Now, when one of displayed messages disappear, the “hole” is covered by shifting other notifications, thus making sure the order remains intact and no message would “accidently” get into the hole. As a picture is worth a million words, here’s (270 Kb) a Keynote demo of suggested behavior and QuickTime (2 Mb) version of it for those who doesn’t have the latest iWork. The idea is also not new and is used in applications like Google Talk.