I think that a vertical layout is more ergonomic than an horizontal layout. Then... Why , the horizontal layout is virtualy the default layout for every desktop out there?.

There is always a lot more of horizontal space than vertical space. Nowdays, even taskmanagers are able to represent every task with live thumbnails (with text) so there's no really need to put text for every task within the panel, so we could use just icons (and icons are icons, right?) to represent any task.

For example. A HDTV monitor has a resolution of 1440x900, so lets assume that a task panel has 32 pixels of height/width...

1440x( 900 -32 )- (1440-32)x900 = -17280

That is a waste of space i think

The mac os' dock is worst!, Every time that i am using mac os, i just put the dock to the left of the screen, and thta is a relief for my

This default is likely because people read from either left to right or right to left in most cases. As a result, it is faster for people to scan across the list of open applications, launchers, system tray, etc.