Going into it - Where does it start?

[Noomz]

I've tried to google for answers on this but I found nothing.
Will I only need to complete Normal diff D3 to start the RoS campaign?

What will it mean for those that might outlevel it by...quite a lot? We'd just blaze through it in no time, right?

[Frozen Death Knight]

If you have managed to finish the original 4 acts already with a character then you will be able to get to act 5 immediately for that character. Doesn't matter if you have beaten Normal difficulty only in Diablo 3 Vanilla, you will be able to do higher difficulties of the new act as well (Torment as well if you have a lvl 60 character on live). Difficulties are no longer restricted to a particular level since all monsters scale with your level and you can switch difficulty at any time.