Let's assume Greece and its private creditors reach a "deal". That deal is far from the final hurdle to preventing a Greek default. According to the Wall Street Journal:
A deal could pave the way for a second bailout package for Greece. However, there have been fresh warnings from euro-zone governments that Greece must improve the implementation of its austerity measures in order to get further assistance. Mr. Rehn has said the euro zone, the European Central Bank and International Monetary Fund may need to inject additional money for a second Greek bailout.
Once a Greek deal is done, an assessment of whether Greece's debt is sustainable will follow. After that, its official creditors--other euro-zone countries and the IMF--will decide how much money is needed to fill Greece's remaining financing needs.
The question then is how many of the €200 billion in Greek bonds will be tendered by private bondholders. If too many hold out, then the debt-sustainability sums won't add up. Greece has said it could then move to force unwilling creditors to accept the bond exchange, transforming the deal from one that could be called voluntary to a coercive default.
Germany also appears to be adding one more significant hurdle according to the BBC:
A leaked plan from the German government proposes a eurozone "budget commissioner" to take control of Greece's tax and spending, reports say. The Financial Times, which has a copy of the plan, calls it an "extraordinary extension" of EU control. Greek Education Minister Anna Diamantopoulou called the German plan "the product of a sick imagination". The European Commission said the budget "must remain the full responsibility of the Greek government". A German official told the Associated Press eurozone finance ministers were discussing the plan.