What is the difference between regular Call of Pripyat?

Gamersgate, huh?

Nothing. You get the lower price if you already have Shadow on Chernobyl.

Loyalty edition also cannot be installed, if it doesn't detect the installed copy of the previous game.

Um, so why? If you're loyal you already have a copy of the game, so it's not for current customers. That means they're driving sales by putting a new name on an old product. This is absolutely unprecendented in gaming ;)

Or Clear Sky.

from Steam:
Loyalty Promo grants a discount off Call of Pripyat if you already own the Steam Version of S.T.A.L.K.E.R. Clear Sky or S.T.A.L.K.E.R. Shadow of Chernobyl. Please note that you need to have Clear Sky or Shadow of Chernobyl registered to your Steam account before purchasing Call of Pripyat to receive the discounted price.

Hmm i won't get the Royalty Edtion because you can't install it as standalone. And the price isn't great they only have like $1 difference.

You're kidding, right?

That is stupid.

Because it is SO HARD to detect whether a user has already purchased another STALKER game previously, right? -_- Dammit.

Yeah, it's not like every online store makes LE available for purchase only to owners of an earlier title there. That would be totally stupid.

Let's just get the entire Stalker series on GOG. Fully updated, and organized so I don't have to go hunting for good mods.

CoP was not automatically Steam right? I have it installed but am not sure if it is Steam or not?

None are, afaik.