Blizzard has released a whole bunch of useful information over the past week or so regarding the MoP expansion pack for World of Warcraft. The most useful of which is probably this survival guide, as it goes to great lengths to explain a lot of the changes, and also links through to other useful information for anyone not up to date with all of the latest news.
For people not yet sure if they want to buy the Mists of Pandaria expansion and have some WoW time still active on their account, tonight at around 8PM (AEST) the servers will come down and the 5.0.4 patch will go live upon reboot 8 hours later (around 4AM AEST).
New content (zones, instances and dungeons etc.) will not be playable, and neither will the new race; that will all come in with the launch of Mists of Pandaria (assumed 25th of September). Until then though, all of the loot, talent and class changes that will be in place in MoP will be active, meaning re-specs are in order.
If you do plan on playing MoP next month, it would be a good idea to log on sometime next week to get any characters you plan on levelling ready, and learn what the spec and talent changes mean for your particular class and play style.