If Outlook now closes properly, you can enable your add-ins one-by-one (restarting Outlook after each one) to find out which add-in prevents Outlook from closing.
You can disable add-ins in the following way; Programs that integrate with Outlook are not always shown as an add-in as well.
It was not referenced as Outlook 2015, avoiding any hint about the name of the next version of Office for Mac—presumably “Office for Mac 2015”.
The only versioning Microsoft has provided is “15.3” as shown in Outlook’s About Outlook menu.
As a result, it's stability also relays on 3rd party software (programs or drivers) - a badly designed piece of software can easily cause Outlook to require more and more system resources, until it crashes or it stops responding properly. If the restart doesn't solve anything, try to close 3rd party programs that run at the same time with Outlook, maybe one of those programs is causing Outlook to require more resources.
If the situation doesn't change, see below for more potential causes/solutions.
GCD puts the responsibility of managing threads on the operating system rather than with the application.Microsoft released Outlook for Mac 15, departing from its history of bundling the application with its Office for Mac suite.Also new was that the release was targeted exclusively at Office 365 customers and not made available as a standalone product.Then outlook freezes up and I have to shut it down and send in an error report.Today, I finally found a solution and am posting it here for my own purposes.