I completely get where you're coming from, Legion fans-to-be (yes, I hold out hope), so let me attempt an explanation. After all, what would Question Week be without the answers?
To me, the Legion has the same draw and attraction other detailed worlds do. I'm talking about Star Trek and Doctor Who, and yes, to a point, the mainstream DC and Marvel universes. There's something special about the geek cred one accumulates by becoming an expert on a truly immense fictional universe. It's where I draw the line between mainstream genre pieces and the truly geeky niches where I want to live. And the more removed that world is from the real one (so if it doesn't take place Earth Present), the better, because you can't take 90% of the reality for granted. The Star Trek universe has dedicated fans in large part because there's a whole universe to explore, and it'll take you 28 seasons and 11 movies to do so before you even consider extracanonical material. When the task is that big (bigger in Who's case), there's geek pride in undertaking it, in mastering it.
So what's the difference between the Legion and other comics? Sure, the DC Universe is (or perhaps I should say was) a big place, a coherent reality where many titles co-exist. And you might feel the same exhilaration at discovering its more obscure corners, or understanding as much of the 75-year-old tapestry as you can. But if you take DC history as a whole, it's too big for any one person to collect and read it all - the Legion is more self-contained and thus, coherent - and the fact that most comics take place in the present (or a well-known past) takes a lot of the world-building away (though DC's fictional cities do help in that sense) - the Legion is its own world and thus, much richer. It's why Legion Lost doesn't really work, whereas I remain committed to the main LSH book, meandering storylines and all!
I understand why people who feel intimidated by Trek, Who or Legion, or just been confused and disinterested whenever they've come in, in the middle of things. But I hope they'll one day hop aboard a Time Bubble and find the perfect gateway into this wondrous future world where everyone wears spandex and knows how to read Interlac (I know *I* do). And together, we might raise our fists and shout: "Long Live the Legion!"