We tried this and found that for the most part people didn't like it. They were closing one notification while reading another, and then would have to follow it.
However, I'm always willing to look at it again.
Also, if you just want to close all of your notifications, you can option+click the close button. :)
I'd like to click the close button and leave my mouse in the same place as other notifications move up to fill its place. The current paradigm right now is I actually have to move my mouse to each notification, which is annoying. Chrome desktop notifications do this correctly, just like their close tab button, which allows me to keep the mouse in the same place and dismiss many notifications.