In terms of leaps I'd say it's roughly equal to the change from 1.6 to Source.
CSS was nothing but a graphical update to 1.6. That's why it was met by a resounding "meh" from people like me who had already played thousands of hours of CS and were looking for something actually new.
So perhaps CSGO will end up being the *third* time we get another CS rehash instead of a proper follow up. de_dust just won't fucking die.