Really though, X-men was a great movie. It was a lot of fun, did a great job of bringing these comic book characters into the "real world", and jump started the whole super-hero film trend.
Justice League just needs solid writing and direction along with competent acting and it will be a success.
It had the virtue at the time
of being the first really serious attempt at the franchise. They got lucky on the casting of most
of the characters (Storm being a prominent exception).
Even at the time though, I remember a lot of "is that it?" type criticism. The FX tech even that recently wasn't up to really showcasing their powers properly, and Singer crafted a film that deliberatly avoided the need for large, "big" scenes becacause of it.
Contrast that with First Class, which, even with all the continuity issues, was the big-scale "super" version of the X-Men that many people were expecting the first one to be.