You've got your snacks.  You're well rested.   

You can't play Legion because of a computer problem?

You don't want to waste time looking for the answer.  You want to play!

Here are some fixes.

1. Check the  @BlizzardCS and @BlizzardCSEU_EN Twitter feeds to see if the problem might be on Blizzard's end.   Patch days usually have a few issues.  Don't monkey around with your own settings if the problem isn't you. 
2. Error 134 (PC) or 154 (Mac)?  Close your game client.  Delete the Cache folder. 
3. Can't get the game installed or a patch to work? Go here: Installation and Patching Troubleshooting
4. Maybe it's a connection problem.  Go here: Latency and Connection Troubleshooting
5. Having a FPS or other performance problem?  Go here: World of Warcraft Performance Troubleshooting
6. Crashing? Go here: World of Warcraft Lockups and Crash Errors
7. Maybe it's your U.I. causing the issue.  Go here: Resetting the User Interface
8. World of Warcraft Item Restoration Tool
9. World of Warcraft Character Undelete

What tips can you share?  Help your fellow WoW players.