His response was useful enough that I'm going to preserve it here (I hope he's not offended by the copy -- since the post is in an archived mailing list, I assume it's not considered private). The following isn't hard-and-fast (people immediately started quibbling with the details, of course), but it's a good rough guideline to the "layers" of the language, depending on what you're trying to accomplish, and is likely useful to anybody trying to learn the language.
It's an intriguing question. I believe the answer will differ a little bit for everyone, but I'll put up a strawman nevertheless. I assume here that programmers have already a good knowledge of Java, so we can take at least pre-generics Java for granted. If that's not the case, some of the early concepts such as classes and exceptions need to be moved to more advanced levels.Later conversation made the point that the levels aren't quite comparable -- for example, L1 is probably similar to A2 in absolute difficulty. But in general, I find this list hugely sensible and very *useful*, especially in providing a guideline of the language features that neophytes are probably well-advised to stay away from unless they're really eager.
Also, I distinguish between application programmers and library designers, because the required skill sets are radically different.
So here's something for y'all to knock down ;-):
Level A1: Beginning application programmer
Java-like statements and expressions: standard operators, method calls, conditionals, loops, try/catch
class, object, def, val, var, import, package
Infix notation for method calls
Collections with map, filter, etc
Level A2: Intermediate application programmer
Recursion, in particular tail recursion
Level A3: Expert application programmer
Level L1: Junior library designer
Control abstraction, currying
Level L2: Senior library designer
Existential types (to interface with Java wildcards)
Level L3: Expert library designer
Defining map/flatmap/withFilter for new kinds of for-expressions
I will note that I, Mr. Armchair Language Geek, haven't yet mastered the A3 level, and I'm nowhere near L3. There *is* a lot there -- but the point is that you can be very productive without needing to know all of it...