Yes, you can download the game for free. But it's still the Free To Play version, which limits the amount of item drops you get, does not let you trade items with people (you can only receive gifts), and most importantly, YOU CAN'T GET HATS. Except for the Gibus and whatever promos you get with other games.
Activating the retail version on your Steam account will add the game to your account -and- change it into a paid version, which removes all these limitations. And gives you a couple of hats.
