Almost everything people experience on a daily basis is electromagnetism. Full-blown control over this force is almost indistinguishable from reality-warping. One could plausibly even pull off some weird transmutations of elements, by reducing the charge between protons to get cold fusion.
It's more helpful to narrow things down to specific aspects of the force. If the power is more like generating an arbitrary EM field, and being able to shape or vary it, then you have something like Magneto, including the things he could do that the movies toned down (force-fields, blasts, etc). If you can mess with ions, the number of crazy chemistry tricks available become too numerous and complex to summarize, beyond pointing out that messing with ionic bonds seems like it should enable the weaponizing of table salt.
Can they emit light? X-rays are the most energy-efficient way to accomplish quite a few things, especially if those things involve killing. How about manipulating light, or the ways in which matter interact with photons? You now have both X-ray vision and telescopic / microscopic vision. If you can interpret radio, two people with these abilities are effectively telepathic. Not to mention all the potential for illusions, screwing with communications technology, etc.
When you get down to it, many, many superpowers are effectively a highly restricted form of EM manipulation, or can be accomplished thereby. Without some sort of specific rules, the answer is "almost anything not covered by the other forces, and you can fake some of those, too".
If rules are required to make such a power less god-like, restricting it in terms of scale, magnitude, or specific aspects (Magneto Vs light-bending) can help. These are still fairly overpowered (being able to wipe every hard-drive within a 100m radius, or lethally X-ray everything within a 100m radius are pretty terrifying).