Probably the most versatile ally in the game and an auto-include in many decks. She is the only non-Hero ally with 3 THW for 1 CD (and one of very few with 3 ATT for 1 CD) which means that in any deck running any ways to heal her or increase her HP, she is godlike as a thwarting machine.

She will be in the first 3 allies included in almost all decks I play, save a few with such a small hand size that they can't play her.

All I can think of when I see this card is how much it reminds me of the Serra Angel from MtG: Revised. It cost a little more than you wanted, but it was such an incredible asset with surprising versatility. Also the wings, I guess.

I won't lie, any time I am willing to have a four-cost ally (which isn't all that often, to be honest) this one could give Fury a run for his money. Snowguard becoming an 8HP wall with retaliate could really solve a lot of problems for some decks.

A 3THW ally could be a literal game changer for an Aggression or Protection deck.

And to be frank, Basic cards are typically a little overpriced because of their ability to be in any deck, which means Snowguard may actually be slightly undercosted for what she's capable of.