I like it. Here are some of the things I noted, warning this is a long list.
1. The gun mechanics feel new and refreshing, but still maintain the basic CS game-play. The recoil eliminates the effectiveness of spraying greatly, forcing you to carefully place shots and learn the quirks of the various guns. I love the feeling you get when you figure out how to use a particular gun.
2. Crouching is more useful now (like in 1.6), walking too.
3. The new grenade types are pretty cool, too. The molotov/incendiary grenade are an interesting addition because of their mulch-functionality. They deal damage, create barriers, and provide a pretty effective cover (the fire makes a lot of smoke). The decoy grenade allows you to simulate a person firing (so an enemy appears on the radar for the opposing team). If used proficiently, you can flank people, cause them to rush the location, or intimidate them to stay where they are (depends on how the person reacts to enemy fire, I guess). Unfortunately, HE grenades aren't as good and, to me at least, have a smaller blast radius compared to Source. You can only hold 1 flash-bang, which is fine by me (be over-kill with all the grenade types there are now).
4. They changed up the maps a bit. Aztec no longer has the back-entrance to T spawn, which discourages camping since Ts can't escape spawn if they're cornered in there (then again, in source all the CTs had to do was just pincer them). Dust is more balanced because they added a stairwell to leave underpass and get outside of the tunnels. They also moved the bombsite closer in CT spawn.
5. Objects in maps are static and can't be moved. This means no more annoying barrels getting in the way on Dust 2. However, you can't move file cabinets or chairs in office...I liked to make barricades

.
7. The new guns are pretty cool. I particularly like the sawed off shotgun for the Ts and the new machine gun (it's fun to use because of it's super high rate of fire).
8. No silencers! I kind of miss this feature because I liked being stealthy sometimes. I heard that this is temporary from someone (apparently, they were causing hit-box problems in the beta).
Well this is all I can think of from the top of my head. I highly recommend the game. For 15 bucks, it's pretty much a steal.