What is the biggest killer of immersion? Bugs? Gold spammers? Addons?
Not even close.
It's the alt-tab combo. A simple keyboard gesture replaces the world of Warcraft which you are temporarily inhabiting with something entirely different. In my case it's usually a Firefox window with blogs that I'm reading or posts I'm writing.
Sure, it's no loss if you're at a loading screen or in the middle of a raid where immersion tends to be thrown out the window anyway. Imagine if you're instead riding through scenic Grizzly Hills. This is possibly the best zone ever, visually, not to mention the excellent terrain design. You're missing it!