I'm sure there's a trivial explanation for this, but here's something that I've always kind of wondered about: Given a single constructor type like "data X = X A B C" can't that be transformed into "newtype X = X (A, B, C)"? There must be some difference, because if there weren't we could transform all single constructor types that way, and dispense with newtype entirely.
Oops, nevermind, I just saw the other thread and link to http://www.haskell.org/haskellwiki/Newtype. Ok, so that seems like a pretty subtle diffenence... I'm assuming the rationale behind differentiating between a single constructor data and newtype is so that data types don't suddenly change their behaviour around undefined when they have only one constructor. I would find example y3 surprising if I came across it in real code!