Have we completed Diablo 3 in 9 hours?

Basically you finished the storyline, in normal difficulty. If you were playing for the story then I'm afraid there isn't much more than that. However if you like being challenged you can :

• Try higher difficulties and push your character to level 60 then earn some Paragon Point
• Try another class
• Try to complete all achievements
• Purchase the Reaper of Souls expansion

Purchasing the expansion will bring you the following features :

• A new Act (The Act V).
• A new class (Crusader)
• Your character can now reach level 70

Upon completing the last Act you will unlock :

• The Adventure Mode with bounty hunting, Nephalem rifts (both bringing endless hours of gameplay) and Greater Rifts for some competition.

You haven't missed anything. If you play fast and do not explore every area in the game thoroughly, you will end it fairly quick.

The game has more difficulties, but if you've done it on the highest one it does not mean you are over.

You can play 5 different classes, 6 with the expansion and for each class you can get different loot that affects your play style.

Diablo is RPG hack'n'slash game, repetitive at first glance, but the rewarding system (loot) is what keeps people playing. You are always striving towards better gear. :)

Either you love it or you hate it.