Another clue is that Clara knows "pond" will be a word that catches the Doctor's attention. While she doesn't seem to have all the memories of her previous encounter with him, she definitely knows things about the Doctor, even if it's subconsciously. The Doctor doesn't believe in coincidences, and neither do the writers of this show, so that has to mean something.
Might River have somehow have escaped the library computer? Or could her love be so strong that something else is afoot? Maybe the Doctor is right, and the universe owes him one? It's all very confusing, but like the best of Doctor Who, it will most likely work itself out in a surprising and exciting way.
Should Clara prove NOT to be River, she better get her stinkin' lips off of the Doctor! He is spoken for!
Either way, though, Clara will be an interesting companion. Just the fact that she has a different perspective from everyone else, declaring the TARDIS "smaller on the outside," rather than "bigger on the inside," as is what other people say, proves that she has something new to bring to the table. Coleman hasn't grabbed me immediately, but given the way the character has been set up, there is little doubt she will leave her mark as a very memorable companion.
This Clara story will likely be a major part of the rest of series seven, but something that has been teased for awhile is the question "Doctor Who?" It's an innocent query in earlier episodes, becoming more common and important during Smith's tenure. "The Snowman" uses the joke often enough that is ceases to be just a laugh. This is obviously part of a bigger trend that will come to a head, and in case anyone missed it before, this special, with its frequent repetition, leaves no doubt that one should notice the question.
Aside from all this fantastic big picture stuff, "The Snowmen" is still a cool story in of itself. With Ian McKellen (The Hobbit, X-Men) lending his voice to the Great Intelligence, the villain is impressive. There are enough twists to keep us on our toes, the snowmen themselves are terrifying, and there's a family at risk at the center of it, the sympathetic heart that a Christmas special needs.