Thanks to chickens_dont_clap for this list of ideas if you ever get bored with Diablo 3.
1) You know there are 4 other classes you can level to 60 and attempt to beat inferno with, right?
2) Being able to barely beat Diablo inferno, and having the gear to put a beatdown on everything you encounter in Act IV inferno are two vastly different things. I look forward to doing timed speed runs of each act of inferno and constantly trying to beat my record.
3) I wanna try out weird builds with every class, which will probably require long periods of time gaining weird gear specific to the weird build. All-thorns Monk. Ranged Barbarian. Melee Wizard.
4) PVP Gearing. Just because it's not out yet doesn't mean you can't start collecting gear for it.
5) Beating inferno with a hardcore character. First I gear a softcore character well enough that he has little risk of dying in Inferno Act IV. Then I play a hardcore character until it has the same stats as that softcore character and beat inferno on hardcore. This will take a LOT of time.
6) Unlock Whimseyshire on all difficulties.
If none of these things sound fun to you then I am very sorry that Diablo couldn't be what you hoped it would be, but it's not the game for you because these things are what it is about.