Thanks to LT for this tip. Below is an explanation by LT of what elective mode is and why it is beneficial to new Diablo III players.
What is elective mode (Short version)
Elective mode is an option that is turned off by default. It is highly recommended that you toggle it on as soon as possible.
Elective mode allows the player to place multiple skills from the same category to their key bindings.
What is elective mode (Long version)
By default, the attack buttons in Diablo 3 are bound to the following: Left mouse button, right mouse button, and the 1, 2, 3, and 4 keys on the keyboard.
The skills in Diablo 3 fall into six categories. These categories vary by class, but every class has six. For the sake of simplicity, we'll be using the wizard as an example. The wizards six categories are: primary, secondary, defensive, force, conjuration, and mastery.
With elective mode off, a wizards primary skill must be bound to the left mouse button, a secondary skill must be bound to the right mouse button, a defensive skill skill must be bound to the 1 key, a force skill must be bound to the 2 key, a conjuration skill must be bound to the 3 key, and a mastery skill must be bound to the 4 key.
By extension, it means the player can only use one skill from each category when selecting which six skills to use. Once they choose a primary skill, that is the only primary skill they can use until they switch it out with another primary skill.
With elective mode on, the player is allowed to place multiple skills from the same category on their bindings. A player could use four primary skills if they so desired. For example:
A basic ice wizard build with elective mode on. The player has selected both two defensive and two force skills. With elective mode off, this would not be possible since the player would be forbidden from using multiple skills from the same category.
In essence, elective mode being off greatly limits your skill choice, and by extension, customization. It is highly recommended that you turn it on as soon as possible.
How do you turn elective mode on?
Once you join a game, simply hit escape, go to options, go to gameplay, and toggle the elective mode box.