Honestly I would rather withdraw the subtraction operators than reopen the discussion about making dict more like set.
+1
I think the "dicts are like more-featured" sets is a math-geek perspective, and unlikely to make things more clear for the bulk of users. And may make it less clear.
We need to be careful -- there are a lot more math geeks on this list than in the general Python coding population.
Simply adding "+" is a non-critical nice to have, but it seems unlikely to really confuse anyone.
-CHB