Overloading methods in R

However, in S3 you can create a "generic" generic function by not 
specifying
arguments but only '...' - this way any methods can take any arguments (and
you don't force your argument names onto other developer's).

So why did they go a step backward in S4 and remov this feature?

I might be misunderstanding what you're getting at here, but if indeed I
do understand this correctly then not only is it still possible in S4 but
I was tought that it was generally considered Good Behavior.

Consider the following code snippet:

setClass("foo", representation(aStr="character"))

[1] "foo"

setClass("bar", representation(aStr="character"))

[1] "bar"

setGeneric("testFun", function(object, ...)

+ standardGeneric("testFun"))
[1] "testFun"

setMethod("testFun", "foo", function(object, x)

+ print(x))
[1] "testFun"

setMethod("testFun", "bar", function(object, a, b, c)

+ print(a+b+c))
[1] "testFun"

z <- new("foo", aStr="")
x <- new("bar", aStr="")
testFun(z, "blah")

[1] "blah"

testFun(x, 1, 2, 3)

[1] 6

Here I've defined a generic 'testFun' and assigned it the '...' argument
(note that you could still specify required arguments,
e.g. 'function(object, anArg, anotherArg, ...)' in the segGeneric
command).  Then when I use setMethod(s) for the two classes I am able to
specify differing sets of arguments for each class.