You can download the Steam Installer here. The larger file (379MB) contains both HalfLife and CS. The smaller file is the online installer, and will install the games you want over a broadband connection.
It seems much has changed, both in game and behind the scenes. Steam Auth is replacing WON as the authing mechanism. Only time will tell how successful this proves to be. Future updates also appear to be via Steam only - not sure whether installation files will be available or not.
In the game itself, the whole look and feel is slightly different. Better console, menus etc. Some maps have bits added. Commands added for quick buying, so no need for buy scripts anymore. Simply bind keys to the new commands. New CT equipment in the form of a Riot Shield. Expensive, but looks fun.
On the server side, it looks like they haven't got the LAN code implememnted correctly yet. A LAN needs to be connected to the internet in order for LAN clients to be able to connect, which is not right. Hopefully this will be fixed prior to our LAN in October :)