Timotheus what about modern Greek then?
How exactly does he defines "urbanized"? I may or may not agree with him you see.
I'd say in general that the biggest differences are
a) simplification of syntax (call it whatever; I mean that for instance the infinitive is "dying" out" )
b) simplification of pronunciation (see a table of modern Greek pronunciation and you more or less have the Koine one)
c) adaptation of the meaning of some words (often because of religious reasons -i.e. aggelos, diabolos-)