I do think many of the maps are unique, but at the same time very similar in varying degrees.
The ways the maps are different are that despite there being a pretty powerful sense of quality control which unites all the maps in a single episode, quite a few maps have very different characteristics in their level design. I can't remember the mapnumbers (or the mapnames) off the top of my head, but there's one map early in the wad I really liked because of all the curved lines and height variation. It's the one with the plasma gun secret in the starting area, and the wide and thin crushing doors. A lot of the mappers have different styles, some maps are fast and linear, some reuse the same arenas for different battles, some maps are very dynamic, and some maps are very specially detailed.
However, the ways the maps all feel the same has a lot to do with the mappers not really knowing what place their map was gonna land in, (which is common in community projects, especially 1024s, community chests, plutonias, and even DTWiD,) so a lot of maps start with the shotgun and some shells at a short distance, and fighting a lot of low level monsters, and progressing onto harder battles with better weapons as you move on. This makes the maps all feel rather indistinguishable from one another as far as gameplay goes. (Still yet, you get what you pay for for downloading an episode that is quite literally named "Get out of my Stations" -- expecting anything other than standard techbases is a little dumb) Despite changing texture themes in the following episodes, a wad with obvious thematic progression also needs a number of "signature" maps too. In the Doom IWADs, this is the crate maze map, the chainsaw secret map, the boss battle map, the slaughter map, the giant crusher map, the barrels everywhere map, the sandbox map, etc. (Containment area; Nuclear Plant; Dead Simple; The Suburbs; The Crusher; Barrels o fun; Industrial Zone)
Of course you would probably want to employ your own map themes too, like a cyberdreams map or a "berserk punch everything to death" map or some other characteristic. If you go back and skip around through the levels, you very rarely start it thinking "oh this is the -something- map." unless of course you know the mapper by name and recognize his individual mapping traits. It makes the whole thing feel like a REALLY dragged out Knee-Deep in the Dead. That's not really a bad thing because they are all pretty fucking great maps. But I think that's what Vermil is feeling in regards to lack of thematic progression.
Last edited by 40oz on 12-11-12 at 20:15