The biggest difference is that, unlike Om, Reagent does not require you to reify an interface to define a component. Instead, components are defined with functions which return a hiccup-like datastructure. If you need access to React's other lifecycle methods, you can provide metadata, but in general, you don't need to bother. Personally, I find this easier to reason about.

The other major change is that state is managed through a special, customized, reactive atom. Everytime the atom is updated, any components which use it are updated. You can have as many of these atoms as you want and you can use them from as many components as you want.