Fix broken Chapter Handling by Base Game
[...]plus the thumbs up menu is annoying as it will interrupt your gameplay every time you want to change to the next map.
THIS! In my opinion it is more convenient to have everything packed together in one place - furthermore telling a story this way is much easier than distributing maps to the Workshop. Moreover assume I create four maps and release them, then I am continuing with the fifth and realize that I want to change the order of the maps... patching everything when there's something new is not the way it should work. And the workshop is definitely a place to release one or two maps to showcase the game.
Ah yeah and the thing about consistency between levels - Half-Life did a good job with the allover 1st person experience without the omniscient player - there are no cuts between the levels and this is how it should be, though Valve did not that way in Portal 2 which is disappointing 
Let's add one think CM already mentioned: story. I have been working for a while on a story and now I want to know, how would you (HMW) tell a story that is shattered into pieces? No one wants to play a game for months!
Ok, perhaps my previous post was not really structured well, but there are a few points I'm trying to make.
The first one is concerning Liam's current project: a mod / map pack, with a custom menu and chapter selection system and everything.
Liam, the part of your post I quoted ("I'm going to make some maps now") sounded to me like your project is still quite early in its development stage, and that it's perhaps not the ideal time to worry about things like the menu system. I would save details like that for later, when I'm (almost) done with the maps. Spending too much time on this kind of thing too early, can really make things grind to a halt. Again speaking from personal experience.
(Then again, perhaps you are almost done with the maps and ready to release, in wich case: forget what I just said 
)
The second thing applies more to these kinds of projects in general, rather than this one in particular.
The notion that players don't have to wait for the next map if they are released all at once, is only partially correct. In reality, we have to wait for all of them. Of course, if you don't know ahead of time that a big mapping project is going to be released, then it's a nice surprise when it does, and the fact that you can play through the whole thing in one go, does make for a better experience story-wise. But "12 Angry Tests" is the positive outlier here. "Backstock", "Portal Stories" and Josepezdj's "Enceladus" series are all similar projects that are in development right now, and not getting released for quite a while, as far as I know.
My previous post suggests that I'm completely against map packs. This is not the case, and in retrospect this was poor wording on my behalf. They do certainly have the advantage of better integration, resource management, and story flow. I agree with you guys that the voting menu of the workshop does take players out of the story a bit.
What I'm worried about, is that mappers bite off more than they can chew, and that they abandon their project before it gets finished, because it turns out to take more time than they expected.
Alternatively, if you start out with an open-ended story (web-comic style) and just begin making and releasing maps, you can call it quits anytime you want, perhaps finish it up with one final boss level, and you still have released some nice stuff. (If you already have a story that doesn't fit this format, then this would not be an ideal solution of course.)
Again, I don't want to tell you how you should organise your project (and if I sounded like I did, then again I'm sorry, that was not intended), but please consider these two things:
- Releasing maps early gets you valuable feedback and publicity. It's easier to keep motivated to continue working on your series, if you know that that work is going to be appreciated.
- Portal 2 is very popular right now, because there is high-quality custom content being released for it right now. Will this still be the case by the time your map pack is finished? (Which may well take a year or more, depending on the size of it.)
So, that's just something I wanted to get off my mind. I will now stop hijacking your thread with these theoreticals, Liam.
(But comments are welcome, as always!)
All the best with your project, and I look forward to see what you come up with!
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
Personally I'm a fan of both approaches. I like the click and run simplicity of the workshop, but i'd rather play collections of maps as a campaign. It would be a lot better if Valve hadn't broken so much stuff with the DLC