NAGS should be simple. I've heard several variations on the story of man who wouldn't play any system he couldn't put all the rules he needed to play on a single sheet of paper (small type and large paper permissible). That's very much a goal for NAGS. In general, there should be as few different mechanisms as possible, but at the same time, the mechanisms should be flexible enough to cover all the situations that could arise. The details might not, but the mechanisms should handle it.
Some games start simple and add huge numbers of rules trying to cover all eventualities. Some games just start complex. Some games start simple and stay that way, handwaving away anything that requires complexity.
My ideal NAGS would have a fractal nature. There should be exactly as much detail as necessary, but no more. It shouldn't take half an hour to determine if you can punch someone. But neither should you be restricted to having to decide a multi-year war on one figurative roll of the dice. You could if you wanted to, but you shouldn't have to.
A continuation of the above idea of fractal simplicity, in some ways. The game should be consistent. Not necessarily with everything else on the market, but within itself. If the numbers that measure character attributes get better as they go up, the number that measures how much damage a spell can do should also get better as it goes up. If one mechanic is used to determine how you open a lock, the same mechanic should be used for how you interact with a computer. The more consistent a game is, the easier it is to learn, and most often, the easier it is to expand, on the fly, with new rules to handle new situations.
One trade off for consistency is sometimes expandability. It doesn't have to be this way. If a system is well designed, it should be relatively clear to the GM and players how you could add rules to cover a new problem. In the ideal system it should be intuitively obvious how to handle a situation you've never dealt with before. The system can then be elegantly simple, and the GM can merely derive all the rules he needs on the spot. Many of the very best GMs can do this with any system, no matter how horrid and byzantine its mechanics are, but if the system makes it easy for even the mediocre GM, so much the better.