I'd met four or five people over time during my grad school career and knew that they were RPGers.
I decided to finally take them up on their requests to play with me. It was probably a mistake, as I didn't altogether think it would work and it didn't. But I created a world using numerous sources and 3 different games to create a workable game. The concept was of an adventurer group from a D&D fantasy through a time well where an alien craft had landed. There were cowboys, aliens and fantasy warriors. The sources I used were from a vast cauldron of apocalyptic settings. They were: Planet of the Apes, Nausicaa and the Valley of Wind, Kamandi and quite a few more I can't remember due to time having passed.
The adventurer team could have had a great time. They were all bright funny interesting people. But all they'd ever done was try killing things. And as a DM I was prone to create discovery and investigation events. The two different styles proved a bad marriage. Adventurers versus Apes with guns versus Cowboys meant that the lethal gunfire made the brave adventurers bleed and die quickly. However it was still fun and I could have enjoyed it more if the fellows had just even once wanted to solve a mystery or try to understand the events they were involved in. But all they wanted was to kill.
I used the rules of Boot Hill and Gamma World to bring all of the levels of technology to the same system. Guns, lasers, swords, and middle fingers all came to be realized in a same system.
The reason I am bringing this up is that I've heard from various people that D&D is a rigid system and isn't a good one to use for anything but low levels dungeons. It isn't about the system. It is about what you do with it.
Creating a setting as a totally alien part of the adventure rather than being a prize to be won is important in my experience to bring the adventure into a place where players can tell their own story but within the confines of a glorious new world. Using direct sources for such a thing is fun and brings out the best in players, who are more than ROLL players.
I should say, having a power oriented group of fantasy characters get attacked and beaten by Apes with machine guns was still notably fun. I just hate that they did things that led them to such an end.