Difference between GM and NM?

It's obviously a no-brainer, but all masters play at a superior rank and it's hard for us amateurs to tell how good they are. For instance, we can't get shown a random master game and know whether or not a FM or GM plays it, etc. 

Well, that's not really true. The stronger the player, the more obvious the difference between a 2200 and 2500. 

Dzindzi, when asked what the difference between an IM and GM was responded, 

Every IM wants to become a GM, no GM wants to become an IM.

How is it obvious? 

*Prepares for lecture about chess stuff I know nothing about. 

There will always be exceptional games, but if you ever watch a game between two GMs, the level of complexity will usually be much higher than a game between two players rated around 2200. In games between grandmasters, every move appears to have a simple, obvious response that is likely to be wrong. In games between masters, the simple obvious move is usually right.

Of course this is an over-simplification, but pull out 10 games played in the 2200-2300 range by both players at random, and do the same for 10 games played by opponents rated +2600. If you use that rubric, you will be right far more often than you will be wrong.

I suspect that a NM is almost as good at tactics as a GM, but the GM is far better at strategy... especially, at recognizing exceptions to the general strategic rules.