Speaking from the female perspective, I'm in agreement with kimbers: go for light.
But also go for style if a girl loves her bike, has a sense of ownership of it (ie. get her to choose it herself) then she will be more inclined to use it often.
My firt mtb was a hideous Halfords monstrosity, £90 and lasted me a week's holiday in Edale. Great for a taster (I thought of it as disposable) but really heavy and clunky, I had to work so bloody hard it nearly put me off! At first I thought I was rubbish, but realised it was the bike. So after that, I became determined, did my research and went for a Scott Contessa 30 (2009) reduced in the sale. I lurrrv my bike. 🙂
Joolz