The changes are subtle, but I think it plays way better than CSS. You really want to be crouching or at least standing still before firing, which makes it feel less spazzy overall. In CSS I felt like you could get kills pretty easily with the assault rifles while running, but now I have a hard time hitting someone while walking and running is basically out of the question, except maybe up close with a shotgun.
They took out so much useless and/or annoying stuff, like the cabinets you could block hallways with, or nightvision goggles, and added a bunch of stuff that I appreciate, like the molotov, which feels really well balanced to me. It costs a bunch of money and a good deal of the time I throw it and accomplish nothing. Once in a blue moon you catch a team rushing in a confined area and cook the hell out of them and it feels well-earned.
So many other things, like tweaks to the map layouts, the scoring system actually taking into account completion of objectives, and separating casual mode and competitive mode. I've played 45 hours using only the matchmaking system and every game was 5v5, normal starting cash, and a decent map rotation, which is great for me because I can't stand having dozens of people playing dust_2 and everyone buying an awp every round.
Honestly, $15 is what a lot of companies charge for dlc that adds a few maps or an hour of single player content, and instead you get a game that has a decent graphical upgrade and subtle to major tweaks to virtually every system, and it went from 1 official game mode to 4.
