Whew. Thanks for the suggestions from dandelO, TheBlackHole and Cyber-Angel.
I tried to soften the clouds and the atmosphere (it seemed to me at one point that the clouds were looking way too blocky); I did this by decreasing cloud density and the hardness of the edges. I tried to have the lightening spark out from a bright spot in the clouds. I hope the overall scene looks somewhat better.
The light source for the lightning is still too bright. Also, you don't need to start a new thread for every version of the image. :) I'd suggest using just a regular sun as a main light source, then putting a second light source (not a sun) in the clouds where you want the lightning to come from. Also, I've never seen lightning come from between two clouds. Try more of an overcast scene.
I like the wispy tops on the clouds. It's the best one yet.
Why not just update the first thread with the new iterations, instead of a new topic per picture? ;)
* TBH beat me to it.