In reading various forums, I've noticed a lot of people don't understand the linear tonemapping exposure controls at all. This is probably due to them having no experience with setting ISO+f/stop+shutter on a real camera, since I suspect most people these days have only ever used an automatic exposure mode on a cheap point & shoot.
On a real camera, these three parameters control more than just the ultimate exposure of the capture. ISO influences noise/grain, shutter influences motion blur, and F/stop influences depth of field. So you really do want control over them individually since they have tradeoffs. But in Lux, these settings ONLY affect the exposure of the render, so there is no real benefit to representing them by their three elements. Well, for people with a photographic background (like me), they make sense, and I can relate them to my real-world camera experience. But again, I don't think that applies to most people. Thus, it strikes me that you could simplify LuxRender for end users by simply making the exposure parameter a single value, ranging from very dark to very bright. The photographer in me sorta cringes at this idea, but I do think it would help a lot of people.
Maybe call it the "linear2" kernel, which would have just the single input value. Thus, people who like the separate values can still use them, and non-photographers can instead select linear2 and have just one slider to worry about. Internally, they can share the same code, as ISO*shutter*F/stop == the linear2 value.