The biggest change will come to Hunters, as the pet leveling system is being dramatically altered. Hunters will be able to specialize their pets using a talent system. Check out WoW Head's Pet Talent Calculator to test out new specs.
Every class should visit their favorite Talent Calculator and get ready to respec. For some classes, especially Warlocks, the playing field will change significantly. So get ready to say goodbye to The Burning Crusade and usher in Wrath of the Lich King.
Tuesday marks the end of Season 4 and will bring on 4 weeks of non-competitive arena. The new Arenas will be opened for players to experiment and battle with their new talents. All earned Arena points will be lost when you hit level 71 on November 13th.
Players will notice Stormwind Harbor and new zeppelin towers outside Ogrimmar are ready for use (although you won't be able to travel to Northrend.) A new guild calendar user interface will be included at the top right of your minimap allowing you to schedule events with your guild
Inscription will be available in the major cities. I've done a detailed guide that will help you prepare to level Inscription from levels 1-350. I've personally have over 2,000 herbs sitting aside waiting to be milled through. If you're not going to be leveling Inscription right away, be sure to find a friend or guildy who is so you can get your Glyphs early on.