It seems a simple idea, right? If a PC game dev allows and encourages user-made mods for their games, those mods should be easy to run, right? It seems this is more the exception than the practice, though.
Now Bethesda got it right- The Elder Scrolls game launchers allow you to add mods by checking a box with the file in the game directory. There are custom mod loading frontends like Oblivion Mod Manager which offer more features, but even the default game loader makes it easy. Even running mods for Doom has never been that hard and continues to be super easy. I prefer the .bat file method of a simple command line for each map setup I run as I tend to run a couple mods together, here's an example
gzdoom doomenh.wad pk_weapports.wad sndckpsx.pk3 roch.wad
This starts GZDoom with no paramaters so I can select the IWAD file for the game I need(Doom, Doom II, heretic, Hexen, etc), loads Doom Enhanced, loads a smooth weapon animation mod, a sound mod then finally the level pack. But if I want to run just a level pack with nothing else I can drag and drop to the GZDoom icon without doing this.
I also believe that running mods for the Star Trek Elite Force series is easy, though there aren't too many mods out there for those games and I never actually tried any but it seems running mods is a simple in-game menu thing.
If you've played Minecraft you know what a nightmare that is. At bare minimum you have to manually install Forge first because Mojang refused to work out a modding API. After that you can drag and drop mods into the "mods" folder, but you're still likely to have to deal with constant config file editing because of block ID conflicts which ARE GUARANTEED to happen when you run several mods together(and it's complicated when the XtraBlocks mod is actually 25 separate mods, not just a single mod, with hundreds of blocks). Minecraft is one of the most frustrating games to run if you want to use mods. And I doubt we will ever get that API since the game sells like drugs even without it. I doubt even Microsoft will bother to have it included now since the game makes money with the programmers doing little real work(and thanks to the TOS and EULA, any user-made mod for the game becomes Mojang's property which explains why so much "new" content with each update was found in mods first)
Come to last night when I found the mod Death Wish for the old Monolith FPS Blood. This is a recent mod and seems intended to work with a DOSbox install, preferably the GOG release. Now I don't actually run the GOG port of that game, it's always given me problems for some reason even though their ports of Duke 3D and Shadow Warrior Run fine with what is essentially the same setup. 2 years ago I chanced upon a custom-made DOSbox pack for Blood which has the game, 2 user mods and BMOUSE all set up and ready to play with no hassle, and it works fine, so I opted to just run that over the GOG pack. But with DW being intended for GOG I installed that again(this on an XP computer, less hassles of running things) and tried to get things working. The usual sound IRQ issues cropped up, naturally.
Then I get to the mod itself which is a jumbled mess of a mod pack. The readme file has formatting issues making it hard to read and the included game config file has errors and formatting issues as well. It seems lots of people are unable to run this mod and I can understand why. I spent an hour trying to run the mod with no success, because the author didn't pack it properly to run with even the intended setup.
Well, after looking at the directory structure of my other Blood install and how its game config and .bat files are setup, it took me a whole of 2 minutes to install DW into that. I merely had to copy the BLOOD game directory, put the mod files in and rename the folder, make copies of a few .bat and config files and edit a couple lines of text in each and the mod worked smoothly.
I think though that if you release a mod and it doesn't even work out of the box, you need to really fix things, which this author didn't. His mod doesn't work "out of the box". I can blame that partly on the oddity of Build engine games(no standardized file pack like Doom used that modders can pack their setup sinto), and the fact Blood's source code was never released for a modern source port, but still, the setup pack the author put up doesn't work. Some people out there were able to edit things to get it to run, none of their tricks worked on my end though and I had to go with a totally different install setup of the game to bypass this.
Running game mods for games that allow mods should be easy, so why is it practically the exception rather than the rule?