Many oil producing nations require $50+/barrel oil to have a balanced budget. After their foreign exchange reserves are depleted (a few years from now), they will start accumulating more debt at their current spending levels, and will likely have to cut public spending. This might not go down so well in some Gulf states http://uk.businessinsider.com/imf-regional-economic-outlook-...
It's not like they didn't have decades to diversify their economies.
What that means for the arab in the street is a different question.