She may be the most powerful mutant in X-Men history, she's one of the few that is labelled "Omega Level". Originally she was a high level psychic with high level telekinetic abilities, a very rare combination. Her psychic powers were so powerful that Xavier put up psychic barriers in her mind until he could help her control all the levels of her powers.
In the comics the X-Men are in a shuttle crash coming back from space, when Jean breaks the barriers using her immense telekinetic powers to save her teammates lives. As her powers continue to grow she is absorbed by them and overwhelmed to the point where she destroys a couple of planets and millions of their innocent inhabitants. Her original sacrificial death is one of the best comic book stories ever written.