This is an extremely hard thing to explain without actually having used FLIR. It's like trying to explain how vanilla ice cream tastes without saying it tastes like vanilla. Try doing it. I dare you.
I have spent years using Gen 3+ night vision. I never understood the FLIR gimic either. I could see perfectly fine at night with my NODs with no problems. But then I used a set. Now...using FLIR to navigate the empty woods at night is nothing special. As a matter of fact the Gen 3+ night vision NODs I had were better. BUT!!! Put warm bodies out in the woods at night and everything changes. My NOD's were good...but the FLIR popped and made the warm bodies litterally stand out from everything like you can't believe.
With night vision, or even just white light illumination, everything is a muted flattened version of reality that you see in daylight. With FLIR, the things that really matter (people, animals, hot running cars, etc) pop out at you like something you can't begin to describe without using the word indescribable.
The biggest problem with FLIR...is you'll never understand its usefulness unless you test the system at night away from streetlights. And what BMW dealership is open at midnight for night-time drives through the country? There is no way for them to effectively demo this tech to make good sales.
Oh, and it pops so much you don't need to stare at the screen. You have the screen in the HUD and when something of significance enters the FLIR field of view you'll be drawn to it. It's kind of hard to describe how intuitive it is.