As Vegepygmy and others pointed out, there is also the GDS (Gamist-Dramatist-Simulationist) model. I personally don't have enough understanding of GNS to know the difference between Simulationism in both models, but if someone has time I would love to hear an overview. Big Model theory is...involved it seems, but I am willing to learn.
Here is Wikipedia's overview of GDS:
"In its most formal sense, the threefold model claims that any single GM decision (about the resolution of in-game events) can be made in order to further the goals of Drama, or Simulation, or Game. By extension, a series of decisions may be described as tending towards one or two of the three goals, to a greater or lesser extent. This can be visualised as an equilateral triangle, with a goal at each vertex, and the points between them representing different weightings of the different goals."
Note that this model describes how a campaign is run by a DM and not how a ruleset is developed by a game designer.
As someone else pointed out, a DM choosing to run a game where the rules say a character must be x to activate item y can make as many simulationist decisions as another DM using rules where characters don't even have x and item y doesn't exist at all.
Regardless of the intent of the game designers, it's the DM who actually runs the game. One of the most basic decisions that determines if a game leans towards the simulationist side has nothing to do with the ruleset at all: are encounters tailored or are they a part of the status quo? Are heroes just as likely to run into an encounter that is too high or too low for them as an encounter that's "just right"?
Theoretically, IMO you could run a campaign using the D&D Miniatures rules for combat that lean more towards simulationism than most campaigns that use 3.5. Consider the following questions:
1.) When it rains, is it for mood or did the DM use a weather pattern table?
2.) If a character dies in the middle of the wilderness, does the party look for a replacement or does the new PC find the party?
3.) Do the PCs receive income from non-combat sources, including investments in property such as strongholds, farms, caravans, etc?
4.) How did you determine if PCs have friends and relatives? How were the friends' and relatives' locations determined? How are events in their lives generated?
5.) If a character is accused of stealing from an ancestral burial ground, how is law enforcement resolved? (And more importantly how do you determine if he is accused in the first place?)
6.) If a caravan is encountered on the road, how do you determine the number of wagons and what's in them?
7.) If there is a thieves guild in a town (again, how is that determined), who is their leader? Are they willing to sell arson and assination services? How do you determine which businesses are under their protection?
You'll have a very tough time finding skirmish combat rules (which is 90% of the RPG core rules) to answer these questions. Yet it's the method that the DM uses to answer these questions that determine if a campaign is gamist/dramatist/simulationist. Does he just make up whatever story sounds cool for his group? Does he tailor things to make an interesting puzzle or scenario for the group? Or does he look for additional, non-combat rules to help him generate a world that's less subjective and biased towards the characters in game and the players at the table?
IMO, whether a DM uses tables and non-combat rules made by third parties (or older WotC supplements) will determine if a 4th edition game is simulationist or not.
