Unplayabale Games
That would be "What is Geist: The Sin Eaters?" Alex. Not because it's silly or complex or broken, but because no one ever runs it.
Complaining aside, Houses of the Blooded is up there for me. HotB is one of those games that's beautiful, elegant, even downright simple at times, but it requires a certain type of player and GM that's altogether too rare to get a meaningfully sized group together for. Both are required to cede a lot of control over their characters to the rest of the folks at the table (which isn't always a bad thing in HotB, even though it's not always a bad thing ever). Both must not only be okay with losing but actually want to lose at things (because HotB is all about losing beautifully, in the end). And both must, through the cosmic forces that align the universe, be willing and able to sit down and actually play the game for longer than an introduction sequence. I know that HotB gets played, or at least it did, but I always worry that I'll never see it done it my lifetime.
Mekton Zeta ranks up there pretty high on the "complex" factor. I think I once heard tell there was a new version being made, but the old version has a wonderfully/terribly complicated mecha crafting system that's sort of underwhelmingly overshadowed in actual play by the simplistic rolling system. So it's a pain.
And then I always forget, no matter how many times I go to approach it, how baffling Exalted is to me. I really want to play Exalted, I swear I do, but I just cannot seem to wrap my head around it. I can run Scion without blinking. I can play nWoD and oWoD with a little help now and then. But there's something about Exalted that I just cannot figure out (or, at the very least, 2e, which is all I've ever experienced). Maybe I just need a better teacher, or maybe I just need to look at it on my own again (he said for the 8th time), but it's just crazy to me that I can't understand the system when I understand so many similar(ish) ones.
There are other systems I won't play, or have no interest in, but I'd say those are the three that fit the nature of this thread. ;)