Bashiok explained some of the Diablo III User-Interface (UI) features, how it is distributed on-screen and more interestingly the intelligent UI engine that auto-assigns an ability or spell to the hotbar when you gain a new one. It is gonna be exciting to see how much the hotbar and UI has changed from what we saw at Blizzcon 2008. Hopefully, it will be playable in the upcoming Blizzcon (August) to keep you updated.
Currently any of your skills can be placed into any of these seven skill spots. The game tries to help you out with their placement though. When you train a new skill it will attempt to intelligently place it for you. So if it’s – for instance – a buff or non-targeted aoe ability (witch doctor Horrify, barbarian Ground Stomp, etc.) it’s going to put it into your hot bar as it’s not something you use through a mouse-targeted enemy attack. For abilities that are used by clicking on an enemy (wizard Magic Missile, etc.), the game will attempt to intelligently place them on your mouse buttons for you. So after learning a new skill it’s already on your bar ready to be used, and it’s placed somewhere that makes sense to how it’s used.
But just because it attempts to help you out by placing skills where they make the most sense, you can put them anywhere you like. If you want to have Magic Missile cast by pressing the first hot bar key (the 1 key on your keyboard), you can, and it will cast toward wherever your cursor is. You can do this with any of the abilities.
The system automatically places skills where they make the most sense, but allows you to customize however you please. It’s a nice wide open, play how you want to play approach, while still offering a helpful guide in the beginning. Also I think it will help, or possibly just not impede, class build diversification.