Storm can work her powers THROUGH Magneto's forcefields.
As we can see, Magneto can take an optic blast and a class 100 strength punch from Colossus with his Shields.
http://img405.imageshack.us/my.php?image=classicxmen01812rq9.jpg
But here we see Storm SLICING and ripping right through him to the BONE WITH HIS SHIELDS UP almost Killing him, all while HOLDING BACK.
http://img405.imageshack.us/my.php?image=classicxmen01813mq3.jpg
Again we see his shields holding up to a full Blast from PHOENIX who is semi-bloodlusted.
http://img394.imageshack.us/my.php?image=shield3hp5.jpg
Whereas here we see Ororo taxing his shield unto certain death until he uses Colossus as a weapon. He even says how much it strained his "magnetic powers"(his SHIELDS)
http://img86.imageshack.us/my.php?image=stormandmagneto1vm.jpg
http://img163.imageshack.us/my.php?image=stormandmagneto26nv.jpg
Also note that given Storm's nature, she was likely holding back here as well.
Poeple often cite Magneto as one of the smartest Supervillains-- a literal GENIUS.
Why would he drop his shields with Nightcrawler and Colossus directly behind him??
It should be also noted that Storm's powers taxed him unto death, while his shields held up to Cyclops, Colossus, Night Crawler and even Phoenix- possesed Jean.