None I suppose...
Already been there and done that with the Tex Murphy games so my GOG experience has been a success.

Perhaps NOLF because I haven't played it in forever (ha).
And probably System Shock 2.
Oh and potentially Syndicate Wars if it comes out here and I'll have the requisite time at hand.

Carmageddon
Grim Fandango
Theme Hospital
Shufflepuck Cafe
Diablo 1 (will never happen)
Jazz Jackrabbit 2
Warzone 2100 (I know)
Need for Speed III: Hot Pursuit
Requiem: Avenging Angel
Future Cop LAPD

Mechwarrior 3
Heavy Gear 2 (drawing problems would probably impossible to solve)

Homeworld
Star Trek Armada
Star Trek: Starfleet Command

Only System Shock 2 would be an insta-buy for me, even if it is sold for more than the $9.99 price tag :)

I used to back stuff up on cd's. A lot of older homeburned cd's started to decay after a few years and are unreadable now. So i'd recommend replacing back-ups once in a while as well. Even an un-used hard drive can start malfunctioning.

Factory pressed cd's seem pretty safe and tape is said to be safe, but no one has that at home.

Worms Armageddon
X-Wing

There are others that would be insta-buy but it's impossible to ever get on GOG, like NFS: Porsche for example.

NFS: Porsche is indeed an insta buy for me as well.

Also: Z (Zed).

1) Worms 2 & Worms: Armageddon -(I bought Worms United as soon as it was available and I do not like that one nearly as much as Worms 2 and Armageddon)

2) Warlords III: Darklords Rising - This one probably impossible to get running on Win 7 though

3) Wizardry 7 & 8 - Oh the hours I have killed with these...

4) Carmageddon & Carmageddon 2

5) Shadow Warrior/Hexen/Heretic/Outlaws - Any of these

*6) Civilization 2/ Civ II: Test of Time

The Secret of Monkey Island

Vivisector is very possible, GOG already has 1C Publishing.

I can't think of any. The only games I would consider "insta-buys" are either games that I can't wait play right now, or interesting games that I'm afraid I will never see again unless I buy it right now.

I can't remember when was the last time I wanted a game so bad that I couldn't wait for it (to drop in price, or Gold Edition release, or whatever). Maybe Wing Commander (in 1990, not the GOG release), or possibly Doom. Thief (The Dark Project) was a close call, but even there I had enough patience to wait for the release of the Thief Gold version before I went to buy it.

For some old and rare retail games, I have many times bought the game as soon as I saw it on some store shelf or ebay (huuto.net here), simply because I wanted to make sure I could play the game at some point.

Thief
Silver
Undying
System Shock

I've forgot The Sims

Not a single mention of SSI's AD&D games? I am disappoint!

But the demand just isn't there. Most people haven't even heard of the game, and many that have don't really care about it one way or the other.

Still, the fact that they have the publisher is better than I expected. Maybe...VIVISECTOR FANS UNITE!!!!!!!

Lula 3d