You won't see many of the original DTWiD authors commenting on this but none would disagree. It's an issue we've run into when we made the first megawad and we know it cannot really be remedied when maps are initially made.
(...) Will have to play through the megawad and then go back to my own maps to re-evaluate if there's "game" in them.
There are and will be some exceptions. Most of the time however the maps are "promising enough", but are not perfect (of course perfect is completely subjective here) in any way. These maps will be discussed, worked on, revamped, modified and whatever else it takes to fit them into the general idea of what this megawad should be. Some maps get worse in the process and fall out of the equation, other are simply getting better.
From my personal experience:
One of my most cherished maps in DTWiD, "Rend of Misery" was first described as being much to homage-y. I tried to fix it, but I reached a point where I didn't think I could do anything to help it. Anything I did was making it worse. The map was removed from the pool, to my dismay :( but it was a good decision. Even in my opinion I've reached the limit in it's potential. I've used it all up and it ended being sub-par.
On the other hand we had the original "City of Corpses", which, to some of us, was one of the most interesting maps atmosphere and layout wise. I still think this map is one of the two most original, among the most authentic ones in the pack (second one would be Alfonzo's "Foundry", or whatever he ended up naming it). But initially it just didn't play great. Gameplay felt random, confusing. Because we were still missing "true" E3 maps a this point, we all decided it would only be fair, to try and modify this "extremely promising map" (my personal opinion on Walter's original work).
I guess what I'm saying is:
We would have to have at least 20-something playbale maps, to see where is it all going, how's the difficulty and progression project-wise. What type (theme, if you will) of maps should we focus from now on, is there any specific theme, that is missing some entries?