Ski, everything you said plays into it. The one important thing you are missing is L/D Max. LD Max is the point where maximum lift is generated for the least amount of drag. On a graph, where the drag line intersects the lift line. That usually occurs very close to stall speed. As we climb, the air does get thinner and we actually are pretty damn close to stall, and therefore LD Max. We just need more margin for safety reasons so we are not right at it. But as close as safely possible. In my example, they are close to LD Max at the lower altitudes but fuel burn at those altitudes are higher. Anyway, I am pretending to know what I am talking about...
. As far as wing shape, it is really more a function of wing loading. The higher the wing loading, the closer to LD Max. The regional jets you see flying around have very low wing loading and therefore are not really efficient at all. An Emb145 will be burning about 3400pph at cruise whereas I can fly a 737-800 and be burning 5500pph carrying 3x as many people. That high burn rate on the RJ is due to their wing loading...of lack thereof. So a stubby fat wing would be slow AND inefficient.