Regardless of your political views, it is almost universally agreed that the large vote for the Greens (and the high number of informal votes) this election was mainly in protest to how poorly Labor performed in certain key policy areas while in government (ie failing to take promised action on climate change, failure to implement resources the tax and then kicking out the PM because of it etc). These particular votes wouldn't have gone to the Coalition regardless.

There is another group of voters who are either swing voters or traditional Liberal/Coalition voters who thought Howard's time was up last election and voted Labor in 07. These voters were also disenchanted that Labor failed to demonstrate leadership in key policy areas, thought Labor failed to live up to the hipe, thought Labor wasted money, thought Labor wasn't tough on people smugglers and/or various other issues, and changed their vote to the Coalition.

Of course we also have our staunch Labor and Coalition voters but they didn't change their vote!

The swing to the Greens was +3.7%
The swing to the Coalition was only +1.9%

Despite preferences, had there been no swing to the Greens, the increase in the Coalition vote would have not been enough for the Coaltion to secure government this election (Remember, Labor had a huge 18 seat margin!).