1. Pyret comes out of Shriram's group's expertise with pinning down exactly what Python and other dynamic scripting languages do right, and (mostly) do wrong. Check out their recent paper Python: The Full Monty: A Tested Semantics for the Python Programming Language http://cs.brown.edu/~sk/Publications/Papers/Published/pmmwpl... for more context.
2. Ruby is far and away not the originator of 'end' to end blocks -- this comes from Pascal and is in other languages that have nothing to do with Ruby, like Lua.
3. Languages that aren't Haskell have ADTs too; it happens that they've lifted an ML-style syntax for defining them.
4. Python is widely used pedagogically, so for better or for worse, students are already being familiarized with the colon, which I agree is a bit anomalous.