I thought this would be a nice thing to do after a conversation with a random in Hell A2 where he was complaining that he couldn't pick the abilities he wanted, and people in the room agreed with him:
There is an option, under the game preferences section of the options tab, to make it so all your action slots are completely customizable instead of being restricted by categories. If you have 2 skills you want in the same category, you can pick and tailor every facet of the abilities you use to fit your play style. I'm going to go out on a limb here and say a lot of people struggling in higher difficulties haven't stumbled upon this yet, and are caught up in a very limiting and restrictive metagame that essentially makes high end play impossible unless you're a god on the keyboard (I don't know how I would play a barb if I didn't have at least 2 shouts available at all times), and blizz really makes no effort to enlighten the player, in fact, the way the game presents it to you makes it seem like you HAVE to play with 1 skill from each category.
So there you go, I hope someone that didn't know this now does.