She doesn't have massive groundstrokes off either wing, but they're technically solid as a rock, very consistent, and she's able to cope with power very well. Her real weapons are her mind and her variety. She's a very adept tactician, surprising for such a young player, and is able to adapt her game to fit her opponent, the conditions, and the surface. She's got nice variety, too - lovely drop shots and pretty decent volleys. A good mover around the court, though I think this is more to do with excellent anticipation than foot speed.
She also has some of the best footwork in the game, which really helps her consistency - she sets herself up for each shot beautifully, and hardly ever looks awkward.
I guess the major weakness is a touch of mental frailty, she's shown a propensity for choking in recent times, though maybe this week will be a turning point.