"What I don't particularly like is that the + operator is used as a concatenation operator as well as an addition operator. The two operations are not at all the same, and using one operator to do both things seems more troublesome than not."
Concatenation has an identity element, and it's associative, making it a monoid operation. I just think of '+ as a polymorphic monoid operation. When the monoid is a group, unary '- comes in to be an inverse operator. When the group is a field, '* comes in to be a multiplication operator, with unary '/ as its inverse.
So I don't have any mathematical objection to using '+ for concatenation, and I actually think it fits quite well. I've even thought about using '+ for function composition, since that's another monoid. (However, recently I've been more attracted to the idea of managing each monoid theory as a separate entity, rather than sniffing the arguments to figure out which operation to use. This is made easy by Haskell type classes, which essentially do the sniffing at compile time based on the static type system.)
