Slices that support negative indexes. List and dictionary comprehensions are amazing: { (X,Y): X2 + Y for X in range(10) for Y in someiter if Y > 5} I love that they become generators when you use () instead of []. Pure genius. Generators from a function containing yield or yield from equally beautiful. % for string interpolation was so good they had to bring it back. Tripple quotes using """ and ''' are hard to beat. Using func(list) to unpack a list into a functions arguments. Or func(dict) to unpack a map by name. Using args and kwargs to implimentaions variadic functions. I love indentation based code blocks. The way 'and' and 'or' can return values is so much better than a ?: terinary. I think I dumped a third of the Python spec there.
I could use a multi-line lambda but it's not a huge omission. Pipelines like bash | pipe or F# |> would be welcome.
go's select statement is also subtle but smart. Doing multiple waits in one thread is tricky to impossible in most any other language.
Built in regex like perl is always nice when you have it.
I'm sure I missed something gushing over Python, but tapping this out over the phone if already killing my thumb.
Java desperately needs groovys's safe navigation operator a?.getB()?.getC(). It's like a normal method call but returns null if the receiver is null instead of throwing an NPE.