As far as gameplay goes, I believe they took a good stab at it. And when I say "stab", I don't mean the metaphorical one that refers to "try", I mean the one that has the intent to kill. The movement is extremely terrible when compared to 1.6 and earlier versions. Also, all the extra visual effects like the dust in dust2 and many other visuals really kill the video card. Even thoough I get 90-100fps in css, many other people complain about performance problems.
It also seems like you can hit the head no matter what. Maybe it's just me but if I aim offset to the actual body anywhere from the chest and up, I get a headshot. Spraying the rifles seems to keep a nice tight spray in the center of the crosshair. Valve was probably thinking, "Hey, let's take away aiming, let everyone just run around with mouse1 down!" When shooting at people, watching the reaction as the bullets hit them is totally different than 1.6. It's kinda hard to determine how much damage you did to someone or where you actually hit them.
Nades also seem to get a big down grade. I nailed a few people right in the face to find they were only taken down to 73HP. Hitting people through walls has been decreased, which some people love. I think Valve is trying to be realistic, but what game do you know that's completely realistic and so much fun? There is a reason CS was successful and Rainbow 6 is not as successful.
It will be interesting to see how leagues are now with CSS, though there is a lot of fuss about it and other leagues may pop up using 1.6 instead of CSS, because CAL/CPL will definitely turn to CSS. Valve will most likely disable 1.6 from being played, just to force people to buy CSS (which is understandable because they don't make any money on 1.6 anymore).
We'll see where it goes. I honestly think all these extra graphics kill gameplay, same goes for the numerous other new games out.