CS:S came out a few years after the release of CS 1.6, but did not find the popularity of its predecessor. Initially, players did not like the spray control system (it was really easy to control), the new graphics (which not all computers could handle) and the new maps, some of which were practically unrecognizable. The gameplay hasn’t changed much (of course, this is a shooter), but it has introduced some minor changes such as auto-purchasing ammo at the start of the round. The graphics, which many complained about, were pleasant for me and did not hurt my eyes like the same ladders in 1.6, and the multiplayer part of the game did not change, and rightly so.
Basically, this is still the same Couner-Strike 1.6 only with updated graphics and changed shooting physics, and also the presence of some new maps like de_season and de_cpl_strike.