The release of the Wrath of the Lich King cinematic has raised the ire of a certain group of WoW players. While I agree that this cinematic lacked the visceral feel of the combat heavy WoW and BC cinematics, I think that it made up for this in its lore.
This got me thinking however. Most of the people who play WoW right now probably picked the game up after the launch of BC, and thus didn't see much of either the old endgame, such as Naxx, or the old world 58-60 zones, such as the Plaugelands (myself included). In fact, it is quite possible that someone who has never played a Warcraft game before has no idea who Arthas is.
It is for this reason that I give the Shadow and Light buyers guide recomendation to the Warcraft III Battle Chest. Playing both Warcraft III and its expansion, The Frozen Throne, will bring you up to speed on the lore that occured only a few years before WoW and BC in Azeroth's timeline. While $35 may seem a bit steep to we who are accustom to paying only $15 for a month's worth of entertainment, I feel that it is definatly worth you while to spend the time to play these games before LK comes out. On a side note, you can probably find it for $30 or less at your local Best Buy, Gamestop, or similar stores.
If you feel that you don't have the time to play these two great RTSs, feel free to get the trunkated version of the lore from YouTube. I recommend this collection of videos for a fast summary.
Edit: The collection that I mentioned above is mostly the cinematics from Warcraft III, which leave a lot of the story out. These movies, however, have pretty much everything on Arthas, if reading up on the Lich King is your main motivation for going back to the old games.