Well look at JavaScript where self (this) works exactly the same way (it's conceptually just another argument to a function) but is passed implicitly and declared as parameter implicitly. People are not very happy with that either...
The problem with javascript's "this" isn't that it's implicit, it's that it acts in a surprising way because of the prototype-based type system. Normally you wouldn't expect a closure inside a method to introduce a new "this" variable, but in javascript it does. Other languages with an implicit "this" variable don't have that issue. In a language with a more traditional type system it's not really a problem.
Making this an explicit argument would remove that ambiguity, and JS isn't the only language to have done things that way, and to have throughly confused people by doing so.
