While Create-A-Class is nothing new to Call of Duty, the way it is implemented is new. Each class you create is given 10 points, and everything you put in that class, from weapons to perks to even grenades will use up those points. It is possible to create a class that has one weapon and 6 perks, which shows the amount of customization allowed.
As you fulfill objectives and kill people, you will gain experience and once you get enough, you will gain a level. It was revealed by Treyarch that the max rank is 55, at which point you will be able to Prestige, which resets your level back to 1 and allows you to moreorless, restart from the beginning (going Prestige will undoubtedly unlock new things, too).
Every time you rank up, you will unlock a new piece of equipment or a new weapon. Also, you will receive a token when you level up, that can be spend on purchasing weapons and equipment. It has been said that reaching level 55 and using all of your tokens will not give you access to everything.
With just the touch of a button and no need for any extra equipment, you will be able to live stream your match that anyone across the world can see. You can also use an USB camera to show yourself in a picture-in-picture window. There is going to be a 15 second or so delay between your stream and the real action, most likely to prevent cheaters. This function is going to be available on all three platforms.
In addition to streaming your matches, you are also going to be able to do live commentary on any match, should you choose. You are able to choose the player you want to watch, go to different areas on the map and even access all of the data (like Scorestreaks) in real-time.
League Play is designed so that players won't be put on teams or in matches with people who might be a lot higher rank than they are, but instead matched them with players around the same skill/rank level.