What I like about looking at Mayu - she has the most beautiful face, she's the cutest thing, often has amazing hair, she has a fantastic body shape, I like her legs, and last but definitely not least, she has an absolutely amazing butt.
I realised they are pretty much all summed up in this pic:
(apart from her legs which are cropped off here)
Also, as you can see in that pic - she has great teeth - a lot of idols have wonky teeth, but Mayu's aren't. Her eyes are quite mesmering there, too.
edit: (there's so much I love about Mayu that aren't to do with her looks - but just thought I'd focus on them here)