As an additional note, the Arena and Rated Battleground Season 11 also ends on August 28th.
1. All raid lockouts will be reset.
2. The new talent and glyph system will be going live, so remember to reassign your talents once you are in-game. Mists of Pandaria Talent Calculator
3. It is highly recommended that you do a full UI reset prior to logging in. Many AddOns will need to be updated; logging in without any AddOns active greatly decreases the chances of anything conflicting with the patch changes.
To do a full reset UI, you will want to save your AddOns in another spot if you wish to preserve them (although many will need to be replaced).
Then, rename/delete or move your WTF, Cache and Interface folders while World of Warcraft is shut down, and reboot your system.
4. As always, it's important to remember to get your AddOns from a trusted source. If you use a search engine, make absolutely certain your destination is not a lookalike site from a sponsored ad that may contain malware. More information on security.
5. While certainly not mandatory, you may find that logging out prior to the patch in a quiet area may make your first login after the patch a little faster. A nice isolated place with as few players as possible will yield the best results.
6. PTR patch notes might be worth a quick glance, many mechanics have changed.
7. MAC USERS: Mac OS X 10.5 Not Supported for Mists -
If you have any questions or concerns about drivers or other technical issues, please feel free to visit our Technical Support Forums.
Mac Tech Forum
PC Tech Forum
Other helpful related links.
Blog Version of the survival guide!
Tech Tips for Patch 5.0.4
Patch 5.0.4 ends support for older video cards
Currency Conversion Before and After Mists of Pandaria
Scroll of Resurrection Update for Mists of Pandaria
Account-wide Pets with Patch 5.0.4
Save Your Chat Logs
World of Warcraft patch 5.0.4 features (wow) (YouTube Video)
5.0.4 Known Issues List
Post-Patch Addendum - CROSS BATTLE.NET TRANSFERS - Pets
Please be aware that pets do not travel with characters across different Battle.net accounts now, post-patch. Caging functionality will not be in until Mists is released.
Once caging is in, you'll have the option to cage up and take with you the pets you want during such a move - or handing them off, but right now, that is NOT an option.