This. Some of the custom maps are much busier and none are nearly as well optimised as the official maps.

Additionally there's level size, geometry complexity, textures, shaders other resources, etc. Just because it's the same engine doesn't mean you can throw just any map at it and the reqs will be the same. You could easily bring down even high-end PCs if you wanted to.

I did some minor dabbling with mods/maps during the Quake-days, and also had a small go at HL1/Worldcraft when the game shipped. Being based on the same engine, the mapping process, tools and restrictions were very similar, and there was an insane amount of tricks you could apply in the editor to make things run better. Everything from scaling textures on "invisible" surfaces, converting solids to entities so they didn't factor into BSP computations, to moving hallways just a few units, could have major impact on your level's framerate. To get optimal results, as Mr. Faliszek says, "you have to know the engine and the tools very well". While there is tonnes of knowledge in the modding community.......

Nobody has the all of the creative talent, technical skills, technical resources, time available and QA that would be applied to an official release. Or for that matter the inclination to do the most boring things like plan.

QFT.
Then again, no amount of optimization can make the aging console hardware catch up with PC specs. This is probably going to be more about compromise than optimization. The mod-makers' main incentive for expending the effort will be having more people able to see their work I suppose, which is pretty strong as the drive behind most non-profit projects tends to be "for fun&glory".

who wants mods for an xbox game anyways? give us episode 3 already!

I guess you could overwhelm either system by putting too many zombies on screen, or too much on screen in general...

This. Some of the custom maps are much busier and none are nearly as well optimised as the official maps.

Nobody has the all of the creative talent, technical skills, technical resources, time available and QA that would be applied to an official release. Or for that matter the inclination to do the most boring things like plan.

One of the problems now when you look at a lot of the mods that are made is they can't run on low-end PCs, they can't even run on mid-end PCs often - they can only run on high-end PCs. They can't run on Xbox 360

eh? Why would mods run any different than the game itself? It's just different content in the game engine...or are they building an entirely new game that needs totally different graphic and memory needs? I guess you could overwhelm either system by putting too many zombies on screen, or too much on screen in general...

Can't run on the 360 hunh? Pshh, I'm pretty sure you'll be telling us how it can run in L4D2 pretty soon though right? Cause that is a totally different engine/game, so naturally those mods will work under L4D2...right? I mean it's silly to think that a $20 game could run the same content that a $60 game can...that is why it costs more, cause it's totally different and better...and not the same at all.