Yeah it's short and easy, but it's only normal difficulty and the beta.
They announced Inferno difficulty, which is the hardest difficulty of any Diablo game. The monsters start 1 level higher than you and go up from there, and you can only do it at max level.
So, you max at level 60, start Inferno, and the monsters start at level 61 and go UP from there. They said it should take months to get to the end of Inferno (to go through all difficulties with 1 character), and there is special gear only acquirable by playing Inferno difficulty, and the gear looks different from other difficulties so that people know where you got that gear from.
So, don't worry about difficulty in the main game.