IMO, like others have said, Grim and Gritty has more to do with the tone of the game rather than rules mechanics. YMMV, but I find the 'harder' things are for the characters the more morally ambiguous they get.
I said something similar on rec.games.frp.misc, some time back:
From: Hong Ooi (firstname.lastname@example.org)
Subject: Re: D20: As Gritty As You Wanna Be
Newsgroups: rec.games.frp.dnd, rec.games.frp.misc
Date: 2002-12-29 05:08:52 PST
On Thu, 26 Dec 2002 07:00:03 -0000, email@example.com
(John Kim) wrote:
>Rick Rauser <firstname.lastname@example.org> wrote:
>>Here's an easy way to make D&D (and any d20 game, for that matter)
>>gritty rather than cinematic. [...] Simply adopt the CoC Massive
>>Damage rule (see CoC d20 rulebook, page 70), which states that if a
>>character receives 10 hp of damage in a single blow, he must make
>>a successful Fortitude save (DC 15) or be killed instantly.
> I'm not really sure about this. For example, I've played
>in 1st edition AD&D which had a number of "save or die" threats.
>However, I never felt that they made the game particularly "gritty"
>in the sense that GURPS or Runequest were -- though maybe this is
>just a difference in the meaning of "gritty".
"Gritty" is one of those words that means what you want it to mean. It's a
bit like "munchkin" in that regard.
I like to define a "gritty" campaign as one where the PCs aren't expected
to solve all the problems they face. By problems, I mean not just proximate
threats like the Lich of the Dark Tower 10 miles thataway, but also the
general tone and theme of the game world. A gritty campaign is one where
the DM presents at least one ongoing threat or condition that can't be
addressed by a group of random individuals, whether by force of arms,
diplomacy, intrigue, or any other methods available to them.
Note that this definition doesn't have much to do with the lethality of the
game. You could have a high-level D&D campaign where PCs die all the time
(and high-level D&D _is_ lethal, in 3rd Ed). However, since the PCs are
expected to overcome the challenges they face (destroy the Dark Lord of
Poo-Bah, beat back the invading armies from the Plane of Pink Smurfs, or
whatever), it isn't gritty. Conversely, you could have a character
interaction-heavy campaign revolving around gangs in a large city. Even if
not much combat takes place, the characters know that most of the people
around them are never going to leave the ghetto, are never going to make it
to greater things, etc. (Whether this latter example is really "gritty"
depends on how much the campaign focuses on social issues. You could just
as easily have a non-gritty campaign set in the 'hood, by de-emphasising
the level of poverty and desperation, and concentrating on action instead
This definition also doesn't have much to do with the campaign's degree of
connectedness with the real world. A Spycraft campaign a la James Bond is
very much "real world", in the sense of not having much magic or ultra tech
(relatively speaking). However, the characters are likely to be larger than
life, and the challenges they face will be those appropriate to the action
movie genre: ones that can be surmounted over the course of a session, or a
campaign. On the other hand, a campaign taking place in the Warhammer
universe will probably feature lots of fantastic creatures like dark elves,
orks, undead and whatnot. It'll still be gritty, because even if the PCs
kill all the bad guys, there are more where they came from: you can't
defeat Chaos, only slow it down.
Personally, I prefer just going into dungeons, killing monsters and taking
their treasure. Much less complicated.