if you isolate her talents each one is okay but i think its the sum of the parts and what she does with them that is her talent plus her ability for pop songwriting/hooks. i also think she is great at reworking/rearranging her songs to give them a different feel.
As a pianist, I don't find her playing to be lacking at all. Her songs don't require immense technical skill. I would imagine that she IS an impressive player and that her pop songs are just not the vehicle for displaying it.