I use a heavily modded version of AD&D 2. I use it because I can create an NPC in about 5 minutes & my players can create PC's in 20 to 30 minutes. Leveling takes 5 minutes.
I've slowly added RM elements to my game over years (even to my non-SW campaigns). I don't like certain elements of RM, but i use elements that are stressed in the SW setting. Thus, for example, I use the Essence/Mentalism/Channeling realms of magic because it seems important to the flavor of the setting. My hodgepodge rules probably would be savaged by OSR D&D guys and RM fans alike.
However designing my own rules is less effort than expecting 30 to 40 year old husbands & fathers with full time jobs to learn an entire new (to them) system or apparently to expect 20 year olds to pull themselves away from social media for long enough to cease having to baby them along instead of reading the rules.
They play Shadow World because of the grand mysteries & intricate "wheels within wheels" conspiracies and plots. I run it as a sort of fantasy Babylon 5 with the doings of mad Andraax, madder Ondoval along with various cults, secret societies, and trans national corporations/organizations as focus of the campaign. It's the big picture stuff like the doings of Lorgalis and the Crown Artifacts that draws me in. I like the deep history: the fall of the K'ta'viiri, the origins of the Earthwardens & the Taranians, the Loremasters machinations, and the activities of the Nameless One. All that works for me.
I could probably run a very good campaign using BRP/Runequest because to me the setting is everything to my players it is much the same. They are the merchant princes & warlords trying to hold against the (inevitable?) triumph of the forces of darkness. I love that the darkness takes many forms & the dark gods aren't the only or even the greatest threat. Evil & good in the setting is not bipolar, but multi-polar. All of this has little enough to do with the game mechanics, but I like getting the hard mechanics of magic & tech so the rules content (which seems very light compared to D&D 3 or D&D 4) is of more value to me than might be expected. How powerful is Schrek? What can a Implementor do? What are the capabilities of Priest Arnak? What are the limits of the Navagator's compasses? All these are answered in a mechanical way that allows me to see just how (un)balancing they are so I find the ratio of "fluff" to "crunch" about right as is. I take about 3 to 4 hours of prep time to convert say the Gryphon Sword adventure in Jaiman including all magic/technological items, monsters, traps & NPC's. I am frightfully slow & thorough.
