IW partnering through Steam is what you will see in the future. Matchmaking is not through Steam, but IW.NET is run in conjunction with Steam.
GSP's will no longer be renting out servers.
No PunkBuster. VAC will be used as an anti cheat.
Retail distribution: Pre-orders will get a disk. Or you could order from Steam.
DLC may now need to be paid for - not confirmed.
Piracy was only a small reason for going to IW.NET - not the whole reason.
Clans:
No more clan servers. Private matches, replace clan servers.
Listen servers will connect to each other.
IW.NET/Steam will replace the community server admin
Competitive Gaming
Competitive gaming has changed forever.
It might be possible in the future for IW engineers to load a "competitive" setting on IW.NET, but it is far from a guarantee. It might also never happen.
Modding
Modding the game is now very much in question. Fourzerotwo had no info on modding. However, it appears that there will be no mods or maps for MW2.
Modding in SpecOps? Unknown.
Today our game has changed for ever.
I'll leave this post with Josh Peckler's comments from planetmedalofhonor.com:
The server community is what's made us. It's what defined us.
IW.NET is getting the PC community to turn into consoles. You're trying to re-write our history.