On the Aesthetic Themes in Portal 2 - v0.9.1
I pay far more attention to lighting than texturing. That's always been my tendency.
And it's not just the "light" part of lighting. I am even more obsessed about shadows.
Lightmap grid density should be adjusted according to the intensity of the light source and the relative distance between the object casting the shadow and the surface that the shadow is casting on. And no, sharper shadow is not always better. It annoys me when I see the lightmap density is increased to "2" everywhere in the map. Not only does this increase the size of the BSP to gargantuan proportion, it's also unrealistic. Just look around at the shadows in the real world. The farther the distance between the object and the surface, the blurrier the shadow edge.
So take a look at all the brush surfaces in your map, and adjust the lightmap density appropriately. Sometimes leaving it at "16" is the best thing. Remember, sharper shadow is not necessarily better.
Another thing, I don't see too many people adding a light entity above the floor buttons. This is done in virtually every single button in the official maps. However, my personal opinion is that the official maps made the brightness of the red light entity too high. I think they set it at 25-50 or so. I think it looks most appropriate at "10", and set it just above the button. This setting will give the button the appearance of a soft red glow, but not too glaring. Some of the official maps set the light entity 64 units above the button, which looks quite odd.
Vengeance
Olympic
[SP] Interception
Interception PeTI
Herman
Uprise
Unified
CrazyGuy and satchmo: Ah, you're psychic. 
Don't worry, it occurred to me that lighting was vital, and it will be coming next update, along with the Wheatley destroyed theme. The little tip you gave about the button light is good, by the way, satchmo, though I can't say I ever noticed it before. Though, CrazyGuy, you were only half-right about the skybox; it is a White skybox, but it DOES give off light. See my tutorial on skybox lighting in the pre-GLaDOS destroyed section for information on how to do this - though your method is perfectly valid, and an interesting alternative.
Anyway, v.1.0 is coming soonish, and all these vital little bits will be added, but I'm kind of tied up with other things at the moment, so it might be a little while. Thanks for all your support, guys!
And it's not just the "light" part of lighting. I am even more obsessed about shadows.
This made me smile a bit. As soon as I read your post the term 'chiaroscuro' came into my head. That was one of a few terms I learned whilst starting my first year at University.
You will see alot more 'chiaroscuro' effects in film noir.
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The connection half-occurred to me, too, but only because I spend ridiculously obsessive amounts of time reading Wikipedia for leisure.
Are you still working on this?
I could use the Wheatley Destroyed Chambers part right about now. 
"I hear voices. But I ignore them and I just carry on killing."
I don't know whether some one has pointed this out, but one of your big spoilers isn't working. Otherwise, excellent guide. The early Wheatley chambers aren't entirely clean, however: in his first "test" there is a walkway poking into the chamber. I would rephrase it to "very rare" damage to the tests. Don't forget to mention to include creaks and groans in the Wheatley maps!
That's what the destruction_ents instance is for. It's a bunch of sounds/camera shakes that patrol around your map on a tracktrain.
[spoiler][SP] Alternate[/spoiler]
You sir are a wizard at mapping.
On the note of the destroyed themes, a few things haven't yet been noted. Camera shakes from distant explosions is used throughout. It is important to use foliage to block out lighting than for the sake of having foliage itself. The feeling you get from vines blocking out the little light that pours in is key to making it feel nature is returning. Instead of using just natural sunlight in a destroyed area, throw in a few artificial lights like the embedded lights. This will help the player see the contrast between the two and also separate the feeling in their minds.