Had a play with this, nothing special. Changes your profile though to the new layout and adds a sort of metagame to Steam for bragging rights with a card and level system. Earn badges from events like sales and you get XP to increase your Steam Level and add show-off stuff to your profile. You can also craft badges from cards by completing a deck, obtained by playing any of these games:
You can obtain up to half the cards for each deck by just playing the game (or idling on the menu if you so desire) but F2P games grant 1 drop per $9 spent on the game and up to 5 drops if you purchased anything in-game before trading cards came in. TF2 also grants up to 5 if you purchased the game before it's F2P transition.
At Level 10 you can get rare foil cards to complete a foil deck. To complete the rest of the deck, you either buy them on the marketplace (yawn!) or trade excess cards with friends which is far more interesting. Completed decks win you custom profile backgrounds, DLC, coupons, emoticons and all grant a badge to show off.
And for those who want to completely ignore it, it isn't required at all and entirely optional. Feel free to donate your unwanted cards to your fellow geeks though...
Hey I rebuilt my gaming PC last night, upgraded from Nvidia GTX 470 1GB to AMD 7950 OC 3GB card, so I'm going to finish off the last few bits tonight and I'll be on Steam a lot hopefully - Leeoc91 is my username - feel free to add
I doubt you will get a COD map pack or expansion pack from it, probably something trivial and silly like a TF2 hat or DOTA2 character.
There's no way they'd make a DOTA hero DLC. Likely outfits, taunts or HUD's though. (Every DOTA unlockable is purely cosmetic and makes no difference to the hero abilities, if you didn't know. It's not like TF2)
I can certainly see TF2 hats being on that list, though!