I've found Subaru to be particularly likable. I had my reservations about the change in primary characters, but I really wouldn't mind the focus on Subaru. Her fighting style is so different from the other characters in A's, except for maybe Signum. It still has the same overwhelming enemies with shear force that we've come to expect from Nanoha herself, but because it's all point blank range, it makes her combat seem even faster. Tiana (I prefer the phonetic spelling over the engrish on the site), compliments Subaru with more technical abilities. She's the brains of the pair, thinking before she acts, unlike Subaru.

We still have the old cast to act as mentors, and they'll obviously get some action. The problem is most of them became so absurdly powerful, Nanoha and Hayate most of all, that seeing them have any trouble with enemies would be wrong. But having them simply waste any opponents would be boring.

The show still has a slow, introductory pace, but the same can be said for the first series until Fate showed up.

I agree that 19 year old Nanoha does look somewhat strange. At the end of A's, she looked fine when she was supposed to be 15. Fate and Hayate aged appropriately though.