It would be nice to support keyword-only constructor parameters, e.g.

@attr.s
class A:
    a = attr.ib()
    b = attr.ib(keyword=True)

A(1, 2)  # forbidden
A(1, b=2)  # passed

Should be allowed for Python 3 only.
Well, attrs should have no Python 3 specific code -- compilation of __init__ function with keyword-only arguments generates SyntaxError on Python 2, maybe enough.

I don't know what name is better: keyword, kw_only, keyword_only.