ABSOLUTELY her serve! When it works, it's great! But more often than not - It's not good enough. Her second serve is almost a liability. In the OGdF final, they were too short and so Serena could attack them. It's almost as if we're amazed when she gets a serve in.
She has enough of a variety of shots - Which she has ably demonstrated to us. But if she could work on that serve, IMO She would get MUCH better results.
Psychologically, she has improved a heck of a lot - But there's always room for improvement. She still tightens up a bit, so mebbe a bit more work there too.