Her problem is that she has nothing to counter a big hitter when the big hitter is on. This usually occurs in a slam in the QF, SF, or Final(but can occur earlier).
When you say that she has "nothing" to counter a big hitter when she is on, what could that possibly mean to you, besides more power? Because she's got everything else: speed, touch, variety and mental toughness.
I think people underestimate Aga's ability to CREATE bad days for her opponents.
Also, I know she is on the run a lot and doesn't hit the hardest groundstrokes, but you'd be surprised how much she actually dictates play throughout most matches, between an improving-everyday first serve, use of angles and spins and solid approaches/volley putaways.
The days she gets hit off the court are when she is unable to dictate play. She's a defender, yes, but if that's all she's doing, she's not going to win a match against a quality opponent.
I think she has started having better results since her first serve has improved. Hopefully, she'll start to get enough free/easy points off her serve that she'll be able to compete against the bigger hitters on the average day.