1) Not as Gear-dependent
If diablo 2 has taught us anything, it's that having a bunch of summons to soak damage and pre-occupy enemies aswell as supporting AoE damage is a way to be effective at even the highest of difficulties with little to no investment in gear.
My estimate is, much like the 'fishymancer', the witch doctor will be one of the best classes with cheap gear
2) PvM strengths
He has summons, healing, crowd control, AoE damage, and debuffs. No doubt he is going to be a PvM beast - particularly late game. And as a first class, for many the main focus will be against the environment and not other players
3) Wealth building
Because of the first 2 points - my estimate is the witch doctor will be the best class for doing endgame runs. Being strong with little gear to begin with, combined with his strengths in PvM will lead to an ability to make the most of 'runs', or 'mfing', or 'farming' in the late game. In diablo 2, the sorceress was the definite top choice for this - however, due to adding a cooldown to teleport, the wizard cannot abuse the ridiculous mobility granted by that spell, giving a more solid and survivability based PvM class such as the witch doctor a chance to shine.
And as for PvP, well all those disables and snares sure as hell are looking might strong