I'm a long time WoW player (started in Vanilla) coming back after a long break. I quit before MoP came out, not because of Pandarens or anything, I just didn't feel like playing anymore. Now I'm going to resub and prepare for WoD. Is there anything I should know or expect? Any big game changes happened after Cata? If it helps, my main character has always been a Horde Shaman.
I really don't know what to say about class changes since I do not have a shaman, but if I were you, I'd check out the Proving Grounds, just to test out how you're doing it in your class.
Tips: Level up to 90, just forget about past tiers, go straight to the Timeless Isle and expect to spend A LOT of time in there til' you're full 496 in ilvl. After that, just run SoO until you have some items from there and go through Flex-mode raiding which is a thousand times better than the Raid Finder. Also, check out some things you missed since Cata, such as the countless scenarios, the Challenge-mode dungeons and Pet Battles.
And don't forget to do a little run in both Krasarang Wilds and Isle of Thunder to check out some of the best content provided in MoP (from 5.1 and 5.2). Anyways, good luck and have fun!