The biggest advantage of the Legion: decades of stories, some of which are still genuinely entertaining, and a vast and varied roster that has huge dramatic depth. 2kt09 is right in that the predated the Titans, but I'd argue that their predating the X-Men also gives you an idea of their legacy, especially since I'd argue that they were being just as crazy as both those premier books in the 80's.
The biggest issue they have is not actually the size of their cast, or the somewhat weird premise; it's the sheer number of reboots, reimaginings, and repackagings. You've got the Silver Age classic series, the Pre-Zero Hour weird-timeline series, the Post-Zero Hour series that built off elements from Pre-Zero Hour, than you've got the Mark Waid version, and finally you've got the restarted Classic Legion but this time reimagined through Geoff Johns.
And part of the problem is that all of them tend to have some really, really cool and engaging creative decisions and stories, but very few have a great one (outside of the Great Darkness Saga from the classic series) and each wound up running afoul of either a creator's opposing visions, editorial problems, or fandom from previous groups not liking having their version dropped.
-Classic Legion evolved with the times and made it to the Bronze Age with roughly contiguous continuity, having characters grow, die, and get married. Unfortunately, we all know that characters growing is anathema to certain creators, and on top of that, DC decided it wanted to make Superman Clark's premier identity, so there was no Superboy to inspire the teenage Legionaires, so...
-Weird Timeline Legion had a lot of timey-wimey-wibbley-wobbley issues; for a time, it's Superboy was a part of a pocket universe created by the Time Trapper, and the Trapper and his assistant wound up controlling the series' status quo. But this became a headache for creators, who wanted to see if they could just restart the whole thing, so they started introducing a Legion precursor in L.E.G.I.O.N., and for a time had two sort of similar but wildly different books, until they used Zero Hour to really start over...
-Post-Zero Hour Legion used the L.E.G.I.O.N. As its backstory, even making Mon-El (or Valor) a legendary, almost deified, character in the background of the universe, making a character who was originally conceived as a "Brother of Superman" gag a cornerstone of the new stories, and the series tried to avoid the cheesy aspects of the original team. It had some strong, DC-In-The-Late-90's writing, but also got maybe a bit too creative, and made some changes that didn't sit with some creative types and parts of the fanbase. Then a crossover with Teen Titans (under Geoff "I love the Silver Age!" Johns) threw some multiverse/Hypertime stuff in there to suggest that the timeline was changing...
- Which lead to Mark Waid's engaging future-teens-with-rebellious-goals version, which reintroduced the cheesy elements but in Waid's reconstructionist manner. It was pretty serious, kind of cool, but burned bridges with some fans of its predecessor, just like the last one had, and it wasn't Silver Age-y enough for Geoff Johns, who'd just risen even higher at DC, so...
-Suddenly the old, old Legion returned in a Superman book. It was typical Johns fair, with somewhat incongruous gore and edginess mixed with genuinely cool moments, and a lot of "Gee, wasn't the Silver Age great?" ideas. This became the new style, but now you had two previous fanbases irritated at the change, and some people felt the new-old Legion wasn't the old-old Legion, ya dig?
-Finally, in probably the only really liked Legion storyline by all fanbases, we had Legion of Three Worlds, a mini series in Final Crises tying the last three version in one story. It was fun and exciting, and even managed to resurrect Bart Allen and Conner Kent. But it also tried to establish that only the new-old Legion was the real one, which still irritated people, and you were again stuck with a somewhat alienated fanbase.
And that's probably why we've had the Legion's return teased pretty clearly in Rebirth, and had them make numerous leaps to television, but still no new book; people are excited at the idea of a new Legion book, but it would be at least the fourth different version in my lifetime.
