Differences in the official IWAD levels?

[Dirk]

I remember being shocked, after only having Shareware Doom, to find an extra switch in E1M1.  Does anyone know why the switch was added?  Are there any other IWAD levels that are available in multiple incarnations?

[Uni]

If I remember correctly, John Romero added the switch to create a better flow for deathmatch. There were more subtle changes to a couple of more maps during development. You can check out this website for a more comprehensive list of changes during Doom's lifespan.