In a very small number of playthroughs of Wrath of the Lamb, I have seen:
- The Cellar (pre-Mom)
- Necropolis (pre-Mom)
- Utero (post-Mom)
What are all the new levels introduced in Wrath of the Lamb, and what's special about them (if anything)?
As idiot6663622 said, the levels and their alternates are:
The only level that has something special about the level itself is the Cellar, which is littered with spiderwebs that slow you and your shots down (you can get rid of spider webs using bombs).
I haven't seen anything of that sort on any of the other alternate levels.
As a general rule of thumb, alternate levels are much more prone to contain alternate enemies, and are the only levels where the new bosses will appear (at least, as far as I've seen).
The floors and their alternates are as follows: Basement;cellar, Caves;catacombs, depths;necropolis, Womb;Utero, Sheol;Cathedral) you can choose between Sheol and the Cathedral.