The varying seasons are not as unrealistic as one might think. Earth is rather unique in having one very large moon by comparison that keeps the planet on a stable axis. If a planet has no moon or multiple moons then it's much less likely to have a stable axis or tilt. It's the Earth's tilt that results in seasons. Stay tuned and you'll find out more about the moons in A Game of Thrones. Not sure if Martin really thought much about the physics of it all, but the idea is plausible. The lack of stable seasons wouldn't have anything to do with the planet's orbit around it's star. Therefore, it would still have years in the traditional sense..., ie, how many days it takes a planet to orbit it's star. Is there magic or other supernatural phenomena? Without spoilers..., yes. We see that in the first 15 minutes. The first 15 minutes deviate significantly from the book which makes it a little less obvious, but that little wildling girl was, in fact, dead at the camp that Will discovered. When Will sees her again in the forest, she's an undead creature called a wight. Martin describes magic best in the HBO specials that led up to the premier. Magic exists, but it exists at the fringes of the story.
