I’ve said before that X-Men just doesn’t feel like Lee and Kirby have their hearts in it, but I don’t think that’s quite right. I think the problem is that it was really their first attempt at building on what they’d already done. It’s a refinement rather than an innovation, pieced together from bits and pieces that worked in their other hits. The problem is that those other hits were themselves still being refined as an ongoing process, and they were way more interesting, which made X-Men redundant.
It had the hook of ostracized and isolated teens, but that was done way better in Spider-Man, the book that laid the foundation of the modern superhero. The team bickered while showing off their super-powers and had Angel and Cyclops competing for Marvel Girl’s affections, but that was nowhere near as good as the strained family relationship in Fantastic Four. They were outsiders in a world that didn’t understand if they were heroes or villains, but, you know, that’s the Hulk’s entire deal. X-Men was the first comic that tried to mash all that up — it’s the first real product of the Marvel Age — but it didn’t do anything better.
Chris Sims wrestles with why the first incarnation of the X-Men didn’t catch fire with the readers, especially in light of the runaway success of the second incarnation. I’m not sure I agree with a lot of what he says, in large part influenced by my Avengers re-read for Wait, What? and noticing many of the same problems in the Lee/Kirby issues of that series (For me, what likely doomed X-Men’s first run was the blandness of the Drake/Roth run that followed, as much as I have affection for it). But it’s good stuff, nonetheless.
One of these days, I’ll sort out my feelings about post-Claremont X-Men and how overbalanced it felt towards the high concept, and write that essay, I swear.