JE Sawyer Mod
(TL:DR - the Sawyer mod makes New Vegas more distinct and maintains interest in combat longer).
In the case of JE Sawyer’s mod, he amped up survival mode (not quite enough - but survival mode is something he particularly fought for.) he correctly saw that people overlevelled the content, especially with DLCs. Combat that is too easy equally descends into tedium as combat that is too hard, and threat makes exploration more interesting. He made different healing agents time release and upped weight restrictions so you couldn’t get into a bad firefight and then burn through essentially disposable food to get out of it.
The weight restrictions are aimed at one of Fallout’s biggest problems as a roleplaying game, intelligence. Essentially the bonuses to intelligence are so huge and the penalties for dumping str especially so low, that an intelligence build could quickly level their way out of difficulty. If weight means more food, and more food means more healing, str builds are more viable in comparison.
Essentially, with no easy out buttons, the game becomes much, much more interesting. All of these changes are to diversify gameplay and focus on increasing the feel of the gameplay. The mod moves it slightly closer to Stalker or Gothic in the living world feel of the game, and a little further from Bethesda’s themeparks.
Auteur Mods in General
I think mods like this and David Gaiders BG T:OB Ascension mod are a bit more than a mod since they focus on changing the experience to better match a creators vision. If they change the difficulty, it is because they’re made by experienced players for experienced players, but not because of a love of difficulty for its own sake.
Gaider’s Ascension mod does make T:OB much harder, but it also changes the storyline to reflect more of your past, and the decisions that certain character’s have made. The point of making the giant invincible is to show that his mother betraying him means more. The point of clearing out dime a dozen demons and replacing them with mini-bosses is to show the villain’s connection to your history.
Difficulty mods tend to focus just on difficulty. I always play BG with Sword Coast Stratagems turned on, because fights are more interesting. I never had bandits try to walk around my web/fireball combo or rogues drink their own potions. In essence it makes bandits more cunning, rogues sneakier, and allows for something new. However, scs mages are terrible; underneath the hood in BG the dev team overlevelled enemy wizards and then took spells out of their spellbooks so the entire game wasn’t a dispellathon or wizard gibbing. SCS has high level mages use every protection in the book, which makes them more challenging and also more boring as four party members twiddle their thumbs until your wizards dispel the last protection three minutes later and your fighters can finally gib them. The creators weren’t just messing with difficulty, they were messing with the feel of the game and screwed it up. I had the same experience with a Far Cry 2 difficulty mod; it led to a brilliant, thrilling 10 minute gun battle through the jungle. But then I realized that every fight in the game would be that way and uninstalled it.
Difficulty mods can make things more interesting, especially if they are like SCS and give enemies better AI, but eventually focus on difficulty for its own sake. Auteur mods refine the feel of the game, and thus make it better.