I like Firefly. It was a very good show and there is so much there that would make it an excellent rpg...but I don't like the Cortex system. I have a version of Cortex but I just couldn't get into it. For me it was too "fiddly" rules-wise.
In the unseen war between DC and Marvel Comics, I am a Marvel guy. I just prefer their universe building over DC. Thus I have all of the Marvel rpgs that have been made...except the one that came out recently using much of the Cortex system in it. Also, to me it seemed like the ability to create your own characters over being "stuck" playing the established Marvel characters seemed an afterthought to the designers. I revolt against rpg systems that don't let me create my own characters.
FATE. I have Fudge (the base game from which it derived) and the Dresden books, but I don't think it's my type of game. I am more for the DM being in charge with the DM setting the universe and the players...playing in it. But I really like some of the ideas for games using the Fate system. Dresden for one and Base Raiders for another.
Conan. This has been seen in 3 different systems; one by TSR, one for GURPS and one for d20 by Mongoose Publishing.I don't own the TSR version and the system itself was widely panned by people. GURPS is not my favorite system despite attempting to run it in the past. d20 is too bloated for my tastes. But playing in the Conan universe sounds like a lot of fun.
GURPS. Talk about having a bunch of cool and interesting books, but not liking the system! I've tried to run their Riverworld book but couldn't get past the system. More fascinating books are Illuminati, Swashbucklers, Hellboy, Horseclans, Wildcards all games I'd like to play but held back by a system that is not to my taste.
I could go on with games such as West End Games d6 Star Wars, Dungeon World, Earthdawn, Elric by Chaosium and more. All games/genres that I like but tied to systems I do not. Basically these have all been games I would like to play/run but the systems keep putting me off from doing so.
Now, I know I could convert these games to systems I do like (despite the many systems I do not like, there are more that I do like), but honestly, that is a lot of work and I'm not sure the payoff is there. Usually if I am buying a setting book it is because I don't want to do the work required in creating a version that fits my favorite game systems. I am looking for something I can use off the rack.
I do however, own a bunch of these games despite the fact I won't be running them with their systems. Often I get them just to read them for fun. Sometimes I get them with the notion of actually doing the conversion work, or at least looking for ideas for creating my own version. Or sometimes I get a crazy notion of running with their chosen system...but that never seems to work out for the best.
Do you have something similar; games/settings you really like but that use a game system you do not like?