Depends on your definition of "breakout".
To me, making the Pro Bowl would be a breakout season. But given his competition at QB in the AFC (Manning, Brady, McNair, Green), that's not likely.
And I'm not certain we'll ever see a "breakout" season. He may explode in year 4 but you'll hear rumblings about how he's a 4 year veteran and was a top overall pick...he should have those types of numbers by then.
So I'm not looking for a "breakout" season. I just want to see improvement.
First thing (as mentioned already) is his TD/INT ratio. If it's "only" even...that's improvement. I'd like to see his rating climb into the upper 70's. I'd like to see his completion percentage reach the 60's.