I'd love to see Python and R, the top two data analysis languages. Obviously this will only appeal to a subset of users, but language interoperability for data work is a hot issue right now.
- R has lazy evaluation semantics (with caveats), and Python is eagerly evaluated. The ggplot library in Python recently posted to HN sheds some light on these issues.
- R has like 2 or 3 class systems; Python has a C++-like class system (without static typing, but gaining it in Python 3)
- They differ in semantics with respect to closures (Python 3 changed things a bit)
- R's built-in types are all vectorized, but that might be a good thing, so you can use Python semantics for scalars and R semantics for vectors/data frames/matrices, etc ?
- Python has decorators, generators, coroutines, etc.
I tend to write my Python and R in a pretty small common subset, but yeah they are in fact quite different.