In Windows I had this program for closing applications by clicking on them with the mouse3-button/middle-button on the task bar. It's just like the function in Firefox where you click with the middle mouse-button on a tab on the tab-bar and the window then closes.
It's not the end of the world if a program/add-on like this doesn't exist for Linux, but I'm so used to this function from windows so it would make the transition a little bit easier for me. Besides, it's a really handy function. Especially when working with a lot of applications at the same time.