I thought I'd spread the message from a fellow guildmate (Strength in Honour!) - credit goes to him.
A lot of people who didn't play beta (and even some who did.) don't realize this is a thing.
Elective Mode is very important. Go to the skill calculator here:
Notice how above each skill slot, it says Primary, Secondary, Defensive, Etc?
With EM off, which is the default, you can only assign those skills to those slots. Which cuts the number of possible builds available to you way way down. For example, with EM off, a Barb can't use both Bash and Cleave, if he were so inclined.
Turning EM on lets you put any skill anywhere (With some exceptions. They don't let you stick some skills on your LMB.), and lets come up with much more diverse builds. And just in general gives you more freedom.
I will never understand why Blizzard set it up this way. During beta, especially open beta, I saw a lot of complaints from people who didn't know the option existed.
Also, turn on Advanced Tool tips. Makes your tool tips actually tell you useful information. The default/simple tool tips in D3 are extremely basic (more than WoWs) and basically don't give you any numbers. That's bad.
So, now you know. (And knowing is half the battle.)
TL;DR: Turning elective mode on lets you place any skill anywhere by removing the skill categories on the hotbar. This allows you to make any build you want.