I think the feature she has that is so alluring are her eyes.
Theres such merriment in those eyes, and she is one of those actors who can emote very effectively with only her eyes.
The scene with her in Hot Fuzz is a prime example of this, as you can only see her eyes yet you immediately recognize her as Cate Blanchett.
https://youtu.be/LIkfBh3gYT4
The commentary by Simon Pegg, and Edgar Wright on the DVD talk more about this as well.