Here's a basic summary from what has been stated at the FAQ:
We value the PC player and have gone to great lengths in providing a more rich experience for our players. We keep our ear near the ground and try to deliver items most requested. The big feature for MW3 we are supporting is Dedicated Servers, this includes a server browser, voting, RCON, and more. Also we’ve implemented some PC only rendering features including SSAO. Lastly we are embracing the STEAM platform by integrating fully with Steam including friends list, STEAM achievements.
Modern Warfare 3 support[s] matchmaking as well as Dedicated Servers, allowing players to find and filter via an in-game server browser or set-up their own Dedicated Server for complete control, without being restricted by any 3rd party hosting requirements. If looking for a Dedicated Server to join, if you choose not to set up your own, you can filter and favorite by Ping, Map, Gametype, Hardcore Variants, Voice, Password, and many other options.
Other highlights include that there will be no lean function and that gamers will have more control over their experience like RCON, Custom Game Saving, NoGFX / Audio Card Requirement on DS.exe, Voice Chat, and SSAO.