7 Sins of Map Design.
I've been wading through some truly awful maps of late, so I need to blow off some steam. Here's a list of the mistakes amateur mappers keep persistently making, to my chagrin.
These are six things everyone hateth, and seven that are an abomination unto the Portal Community:
#7 Him that doth not think of sequence-breaking solutions.
#6 A deceitful level design that offereth unintuitive solutions.
#5 Reflexes that are swift being required.
#4 Minds that devise to leave empty space.
#3 Levels that trap innocent players.
#2 A plain test.
#1 Misaligned textures.
Did you know that American English isn't even my first language? It's actually [spoiler]British English[/spoiler]
Released maps of note:
[spoiler]Versatile Diversity
L-Circuts[/spoiler]
Mazes, rollercoasters, bad placed models, wrong textures, 128 hallways, bad optimization, crawling tunnels, useless puzzle elements, trapdoor, no fog, no lighting, no cubemaps, no texture variation.
--Lights with no source.
--Bad integration of HL2 concepts. Possibly any integration of HL2 concepts.
--Lights with no source.
Now with 100% more inline comments!
Ahem gravitygun.
-Overconfidence in mapping skill
-Unwillingness to learn
-(Improper) use of vertex or model textures
Aperture Science: We do our science asbestos we can!
I was thinking more "OMFGZOMBIES I SHOULD TOTALLY PUT TWENTY OF THEM IN A TOO-BRIGHT ROOM WITH A METROCOP AND...UH...NON-FUNCTIONAL VORTIGAUNT YES VORTIGAUNT"
Now with 100% more inline comments!
lol I see... That would indeed be one of the worst things ever. 
Aperture Science: We do our science asbestos we can!
You say that as though it hasn't happened.
Now with 100% more inline comments!
You say that as though it hasn't happened.
Really? In a Portal map? Pictures PLEASE 
... or better, links!
useful tools and stuff here on TWP 
[spoiler]
[/spoiler]Yes, those are indeed specific symptoms of mappers taken by the seven sins.
I was thinking more "OMFGZOMBIES I SHOULD TOTALLY PUT TWENTY OF THEM IN A TOO-BRIGHT ROOM WITH A METROCOP AND...UH...NON-FUNCTIONAL VORTIGAUNT YES VORTIGAUNT"
I can think of ONE instance where that worked: Anyone remember Hetzchase Nailway? It worked there, but because that mod was already an acid trip, so such a jarring shift in mechanics and tone actually seemed quite fitting.
Did you know that American English isn't even my first language? It's actually [spoiler]British English[/spoiler]
Released maps of note:
[spoiler]Versatile Diversity
L-Circuts[/spoiler]
I just LOVE every single bit of Hetzchase Nailway 
useful tools and stuff here on TWP [spoiler]
[/spoiler]