6. Bran And The Three-Eyed Raven
Okay, so we'll almost certainly return to Bran, Hodor, Meera Reed and those odd little Children of the Forest again at some point, but for now they've been left by the wayside.
When last we saw this motley crew, Meera's brother Jojen (still alive in the books incidentally) had met a nasty end, and Bran was preparing to learn how to "fly" from the mysterious Three-Eyed Raven. This is more or less where we leave off in A Dance With Dragons, and even though other characters have progressed slightly beyond the point we left them in the books, the decision was made to stop Bran's journey there.
Showrunner David Benioff has compared Bran's Season 5 hiatus to Luke Skywalker's off-screen training between The Empire Strikes Back and Return Of The Jedi, suggesting that the young Warg will return with some impressive new powers and a lightsaber in Season 6. Well, maybe just one of those things...