I bought the Orange Box on Steam as a download about a year ago. I really like being able to log into my account on any computer and being able to re-download the game and play it without keeping track of a disk or DVD key.
Now Portal 2 is coming out. If I purchase it on DVD (I have some Amazon.com gift card I want to use) do I loose any of the cool Steam features that you get with a totally digital game purchase?
If the game is an official Steamworks game there is no difference between the DVD version and the online version, the DVD is just like preloading the game. This means you get all the advantages and disadvantages of Steam, no matter where you buy it.
This is not necessarily the case for games that are just sold via Steam, but don't use any of the Steamworks features.
As Portal 2 is produced by Valve, the same people responsible for Steam, I'm pretty sure it will be a full Steamworks game and it won't matter where you buy it. I couldn't confirm it though, so I can't be 100% sure.
You can generally register all Valve games on Steam and add them to your library, so this will most likely also be the case with Portal 2.
If you have a PS3, you might want to consider buying the Portal 2 disk for PS3. This has the additional benefit of allowing you to play the game on your PS3, and Valve has said that PS3 owners will be able to download it to your PC as well, so you don't lose out on anything. If you purchase it for your PC, you will very likely NOT be able to download it to your PS3.
There is absolutely no difference with games that use Steam for DRM. You enter the CD key into Steam, and the game is added to your account.
You can then either install from the disc, or just download it from Steam's servers, like you're used to.
Note: this only goes for games that use Steam for DRM, so games that have a note about Steam on the disc cover. (Like every Valve game for example). Games that are simply distributed over Steam, but with their own DRM don't always work like that.
It depends on the game.
Since you specifically asked about Portal 2 though, it's probably safe to assume that (like all other Valve games dating back to Half-Life 2) the boxed version will contain a code that you enter at some point during installation which adds the game to your Steam account, giving you the benefits of Steam ownership without the long download time up front.
