I hope the Greenlight team will approve it and make it available through Steam before the release date. That would make everyone's life much more easier and gain a lot of publicity for the mod (and team (and overload the Steam servers)).
I'm also curious to see how they will conclude the game by ending as you enter Xen. Pretty big leap to remove Xen which provided that eerie atmosphere like in the original Half-Life. Possible sequel (that includes the full playthrough of Xen)?
Well, I know how they end the game. I don't think I should say it though, it might ruin the method in which they do it.
I'll just say that the game ends when you enter through the portal to Xen in the most literal sense.
It also wont be on Steam by the release date. I don't think anyone other than the Developers or Valve will have any idea when, but I read on the official forums that it will be late by weeks or even months.