after working with 3ds max and vray at work, I think lw's image viewer could have a few changes that could be helpful while we render with F9
One would be to keep in memory the last render (with an option to deactivate it).
Another would be a render history (I know we have one or two third party plugins for this).
Then the Limited Render...in vray frame buffer we can have the last render and keep rendering limited regions and these limited regions stay over the last render, this way we can keep changing the image by fixing the areas that need to be fixed and in the end we have the final complete render. In lw we can render limited regions but in the end to have the final render we have to render the whole thing again.
another good thing would be sliding handles to compare two renders. In the history list we could choose two renders and have horizontal/vertical sliding handes to compare the two. These handles would be next to the render and not a slider like we have now that you have to open the image control panel to access it.
for now this would be nice.