More than the interface (which is a matter of taste; I prefer single-window), I refer to the experience as a whole. I've used only Lightroom (as Adobe product)

More than the interface (which is a matter of taste; I prefer single-window), I refer to the experience as a whole. I've used only Lightroom (as Adobe product), and it's hard to define what casual editing is, but the basic enhancement functionalities were easy to use, and especially, they were very effective (again, for a casual user). It had, as far as I remember, very good automatic enhancement, that one could use as a base. The other products I tried weren't so effective. If, as I suppose, this type of functionality requires a significant investment to develop, then it's out of reach for non-very-well-funded open source development. Dehazing was another typical example of LR functionality: it's stupid simple (a slider), and does effectively what's supposed to do for a casual user.