Saturation is actually dropped quite significantly. The reason the original came out so gray is because the 'scene' mode I use on my 500D gives me the most accurate representation of the raw data I'm going to have to work with when I get the image onto a computer. Another part of why it's that gray in general is because I was taking this through the dusty front windscreen of a car.
Anyway, as for the processing, I don't have a physical GND so I applied one over the clouds to darken them a bit over the rest of the scene - this difference was less than 1 stop. I did increase the contrast, and boosted the highlights slightly so that they would stand out more to better represent the kind of 'bloom' we saw.
I used a light GND along the bottom to darken the foreground's road slightly since I didn't want it to draw focus away too much. There's a light brightening GND from the left to better portray the light that was spilling over the hills behind us.
Once I'm satisfied with everything I apply a very light vignetting effect since no lens in the world at its designed-for imaging plane size is 100% devoid of vignetting, and most people growing up seeing photos are used to vignetting and consider it a 'natural' visual element to photos. The vignetting that was caused by my lens was correct for since I had to crop and rotate the image.
Vignetting, or selectively brightening and darkening in the photo (aka dodging and burning), can have a great impact on how the viewer's attention is drawn through a photo, and is often a crucial step of processing/printmaking to ensure things come out how you wanted them to. Especially in landscape photos, having absolute control over the cloud cover, sun's position and where you're able to apply or remove light simply isn't practical, and so it becomes a crucial step to getting what you wanted out of your image.
Ansel Adams' work is a great example of this.
Here's another version of the processing with all the GNDs and two dodging pins visible, and lines drawn to show where they're located and going. The circled pins are the dodging pins, the arrows are all GNDs.
And here's what the settings look like. The original processing I had of this didn't involve nearly as many changes. I revisit the image frequently to try different processing on it.
