Adding the + operator for dictionaries feels like it would be a mistake in that it offers at most sugar-y benefits, but introduces the significant drawback of making it easier to introduced unintended errors.
I disagree. This argument only really applies to the case "a = a + b", not "a = b + c". Making it easier and more natural to produce code that doesn't mutate in place is something that should reduce errors, not make them more common.
The big mistake here was * for strings which is unusual, would be just as well served by a method, and will ensure that type errors blow up much later than it could have been. This type of mistake for dicts when you expected numbers is a much stronger argument against this proposal in my opinion. Let's not create another pitfall! The current syntax is a bit unwieldy but is really fine.
/ Anders