Here's a short list of games that I will never DM/GM, for any number of reasons.
AD&D 1st Edition - Confusingly organized, even more so than 2e. I do like the tone of 1e much more, though, so there's a slight possibility that I might run it with a heavy dose of house rules to remove some of the dumber things (like alignment languages, material components, and the massive mess that is the unarmed combat system). I recently met someone who loves DMing it, though, so I wouldn't mind being on board as a player.
GURPS (any edition) - I tried to figure out the 4th edition "GURPS Lite" document. I still haven't. I'm not a big fan of point-buy systems in general, and this one is just way too fiddly for my tastes. I would probably not play this one either.
Ghost Dog: The Way of the Samurai - I love this book (not surprising, since I loved the movie), and the Tri-Stat system is a bit less complex than GURPS in terms of point-buy. I'd imagine that in play it's pretty fast and simple, but the problem is that I have zero ideas for a modern-day crime game. I certainly wouldn't mind playing it, if someone else had ideas.
The World of Darkness - The system isn't too bad, and my brief experience playing Mage: The Awakening would help me get a handle on things, but I only have one story idea. I'd hate to make everyone make human characters for a mini-campaign, but if players would be up for it, I might be convinced. I wouldn't mind playing, either.
Star Wars (any post-WEG version) - I don't know nearly enough about the universe to make for a satisfying game for the players. My entire experience with the franchise is the movies, some of the video games, and a couple of kiddie novels I read as a youngling. I obviously wouldn't mind playing, considering I already have and had a blast.
Pathfinder (the full edition) - Nope. Way, way too much crunch. Wouldn't mind playing, but I will never upgrade to the non-Beginner Box edition as a GM.
You might notice that D&D 4th Edition isn't there. To be honest, if I had the time or the desire, I wouldn't mind DMing 4e, but I doubt that most of my players would want to put themselves through that. HackMaster 4e is also not there, because as much as I would want to run it (and endure all of the rules arguing and struggles thereof), it's based on the AD&D engine that two of my players hate so much, and few others would want to struggle with the rules.