Diablo 3 comes with a smart and very comfortable UI for the abilities of your character. The bottom bar has only six abilities, two of them (the ones from the start of the game) being used by left and right mouse click.
The other first four abilities will be assigned to the first four keyboard key numbers, 1, 2, 3 and 4 as you progress in the game and unlock these abilities.
There will be plenty of discussion in Diablo 3 strategy guides on how to arrange your abilities for playing the game. But no matter how abilities are arranged you will want to be able to select them as fast as possible.
How to use keybinds and how to play Diablo 3 only using the mouse?
This can be done just like in World of Warcraft where the player can assign abilities to mouse scroll wheel up, mouse scroll wheel down and mouse scroll click.
These are three abilities that can be used from the mouse. So, only one ability will remain to be pressed from the keyboard.
If you have a gaming mouse with more than the three default buttons (left click, right click and scroll click), like a Razer Naga or Razer Naga Hex, you can keybind those extra buttons for the Diablo 3 abilities and the keyboard will become useless.
Follow these steps in order to keybind the first three abilities to mouse scroll wheel up, down and the scroll click.
Login inside the game, click on the game menu icon (the computer from the bottom bar), Options, Key Bindings, scroll down a bit, notice where is saying “Action Bar Skill 1, 2, 3 etc. Click on “Action Bar Skill 1″ and do a mouse scroll wheel up movement to assign it.
Repeat for the second and third action bar skill using mouse scroll wheel down and scroll click.
This will make the game more comfortable to play.