Greece could be run by a bunch of Marxists and other socialists
The 'traditional' Greek parties have reaped the results of their politics:
If Syriza wins most of the votes, they will get a hefty 50 bonus seats, thus making sure they can rule the country. What will happen next is completely open. The most likely outcome is that they will terminate the austerity program.
A hard default would be the consequence since Greece could not stay in the Euro zone even if they wanted.