Personally, I feel that grittiness is all about atmosphere and DM style, and not about nerfing PCs or low magic gaming. I love gritty gaming. But I will NEVER play Warhammer after having tried it once.
If Yoda was giving advice to Luke on gritty gaming, the conversation would be something like this:
Luke: So nerfing PCs and lowering the magic level is the best way to make a game "gritty"?
Yoda: No, no. Quicker, easier, more seductive.
The core essence of "grittiness" is two-fold. It is threat of death, and the remoteness of victory. Not no chance of victory, but victory should seem almost but not quite out of reach. And certainly not attainable without sacrifice. And that sacrifice can come in story form.
The grittiest game I ever played in was a 40th level epic 3.0 game. That's right, a 40th level Epic D&D game. That game was both grittier and more fun than the one experience I had playing Warhammer. It was all about the DM.
That game oozed darkness and atmosphere. The PCs were not nerfed in any way, we had our full complement of magical items, etc. No house rules against the PCs.
However, D&D as written doesn't really lend itself well to gritty gaming, so one of the big mechanical changes the DM did was to make our opponents tough, and unrelentless. The game was very fantastical and epic in all areas, but it was very dark and grim as well.
Our party was part of an elite strike force making commando raids against a mountain fortress in a kingdom ruled by an ancient vampire lord. This guy had legions of tens of thousands of beastmen, platoons of giants, badass high level vampire lieutenants, etc. For all our power, we never once felt like superheroes. When every giant you encounter has 20 levels of Fighter and when Winter Wights attack you in groups, you feel very mortal.
Sure, I had all the options of a 40th level character, but yet it was a constant brutal battle for survival against overwhelming odds. The Warhammer game I played had the same constant brutal battle against overwhelming odds, but far more restrictions on my character, and far fewer options in a fight. One game was fun, the other was not. One game was over the top and fantastic, the other was "realistic". But as far as grittiness goes, the "realistic" game had nothing over the epic game. At least in my opinion.