I decompiled most of their work for Portal 2 and noticed how HORRIBLE IT IS. There is overlapping textures everywhere, they ultimately refuse to optimise to the fullest quality possible and they even have floating models IN PLAIN SIGHT.
They even texture the brushes on the outside in a lot of the places. I don't know what they were thinking because this decreases performance, and they way the make up to this is by putting areaportals and hints everywhere. Hints/skip should be placed in the most ideal places in order to split the leafs well, rather than putting them everywhere so then it could work 'reasonably well'. If they had used 'nodraw' on every single face that is not visible by the player, the FPS would probably triple and people with worse computers might not have as much lag when it comes to rendering the models they absolutely spam everywhere.
I noticed that Valve usually keeps the textures aligned well, but only in parts that are snapped to the larger grid sizes. In the coop lobby, the wall behind the player spawn, there is a mis-aligned texture which could have been easily fixed by changing the texture x and y values both to 0. On the walls in the same area there are also textures that obviously do not make sense as they are cut half way through a panel-wall texture (i.e. mis-aligned as well) which makes the lights next to it seem plonked in.
Even on the roof of that area, they didn't bother taking two seconds to split a brush in order to make it look 10x better and changing the texture to something more reasonable, like a trim texture.
Mis-aligning textures just shows how rushed they might have been, rather than focusing on the quality of the map. Not only does it look terrible, but the fact that they have done this multiple times in PLAIN SIGHT is what gets me.
When it comes to overlapping textures, Valve just loves doing this. Usually it may be in spots that you cannot see (or is hard to see), but this, again, dramatically decreases performance as it is trying to decide which texture to render when they are in exactly the same area. I noticed that using instances wrong creates a lot of overlapping textures as Valve usually does not cover the outside of their instances with nodraw.
I know that the instances might be 'flexible' in the sense; but no-one is going to see the outside of a spawn dropper, or the opposite side of where you walk out from in the coop_spawn instance. Minimize the amount of faces rendered = better quality play.
I have noticed that Valve refuses to put player clip in obvious places to stop the player from reaching places that they shouldn't have. A good example of this is how some of the good Portal Speed runners find these mistakes and use the glitchy areas to take a short-cut through an area which should have been closed off. You know when you are not supposed to be somewhere because you can obviously see nodraw textures on blocks, and events get triggered wrongly/not at all.
Again, in the coop lobby, they haven't put clip on the holes in front of the 'arm' looking models so you can stand on these brushes and look behind them (of course there is a nodraw texture and you can see through the wall). I'm not saying you should put player clip everywhere, but in parts where the player can physically stand or stand near.
When I was playing in the lobby with a friend we noticed this: 
.
If they moved the model back 32 units or so it would be on the wall, and not floating. In fact even when you fall down and die in the hub you can see all the underneaths of the models. A 'fade' would be good here.
The only thing I can say that I like about Valve's work is the amount of automation when it comes to the scripts called. This is very productive and reduces the amount of entities placed in the map ten-fold.
Overall I give Valve a 1/5 for their dodgy, rushed work. If they paid their level designers more instead of hiring expensive actors for a few simple lines then surely it might be better quality. Then again, their work is really similar no matter what game you look at.
