hiya folks. I bought this game and its DLC at the same time and was finding it to be quite the motherfucker. Found out Wrath of the Lamb does increase the difficulty from moderately more difficult to severely more difficult depending on the layout you get. Which i'm sure is old news to all of you.
HOWEVER, something that is not old news is a mod Steam user DosBoss put together that lets you choose whether to launch your copy of Isaac with or without the DLC in play. I tried it just now and it works exactly as advertised. Check it out: