I have regularly looked like this --> over DW casting decisions, but pretty much without fail I've been perfectly happy with the way they've turned out in practice. So my official policy is now to be all Zen-like about it and wait and see.
Admittedly, I think they should have some companions who are not from 21st-century Earth, but it is Official BBC Policy that they must fall into that category because audiences are too dim to cope otherwise (OK, I may be paraphrasing a little there). Within those parameters, and so far as one can judge from a photograph, she looks fine (possibly even has a hint of the Sally Sparrows about her?)
But please, not another "Doctor in Love" story arc. It's clear that that's what RTD had in mind (from reading A Writer's Tale, he had a romantic subplot in mind before Catherine Tate signed on for a full series and dislodged it) but I expect better things from SM. OK, he does have an urge towards the romantic relationshippy in his plotlines (Madame de Pompadour, River Song), and I'll allow him a certain amount of that, but not more Doctor/companion stuff, please. If I want that I can read Kay's fanfic...
So I suppose I'd feel marginally more reassured if they'd gone for "matronly" or "has a second head" that they would be avoiding those areas, but the fact they've cast a 21-year-old doesn't automatically mean they are going for Rose Tyler Mark II. I hope.