It is not that Ohio is particularly important state. Most states already have a huge bias one way or another. The South & Mountain states prefer Romeny. The Northeast and west coast prefer Obama.
So there are relatively few states left with large numbers of electoral votes that are close contests. Ohio is one that seems to play this role all the time.
One of the best ways to look at it is go here:
http://www.270towin.com/
Most of the states are pre-set to be their nominal tilt (blue=Obama, Red=Romney).
Click on Ohio and see how it controls the election depending on who wins.
Of course other states can also play this role such as Florida & Virginia. But Ohio is often the one that has a very close race.