Thank you both very much for the compliments!
You're right about the outlines along the desert rivers - and besides suggesting fertile zones, they help make the rivers contrast more against the bright desert colors. The rivers themselves are a combination of Wilbur-generated rivers and the ones I had drawn before creating the heightmap. Most of the rivers in the north are from Wilbur, though with somewhat exaggerated squiggles. I admit I may have gone overboard with the squiggliness, probably to avoid drawing them too straight, but I imagine that a lot of the northern forests are fairly swampy, not unlike much of Siberia. Given how much trouble Illustrator was giving me while I traced the rivers, I'm inclined to leave them be.
As for the texture compliment - I'll look into writing a tutorial. It's mostly a generous use of gradient maps, along with a layer of difference clouds set to Overlay, but the exact choice of colors for each biome took a while to get right.
I appreciate your vote of confidence on the coastlines! As for adding a bit of noise, I've given that a shot in my latest WIP below. The effect seems a bit too prominent, even after turning down the noise layer's contrast, but I may be able to get it appropriately subtle with a lower opacity.
### Latest WIP ###
zashave_biomes_gradual.jpg
The main change here, aside from the extra noise, is something Robulous recommended: smoothing out the climate transitions. Since the climate visibility is controlled entirely by alpha masks, it was fairly easy to just add a wide-radius Gaussian blur to the gradual transitions.