Leroux: Blood Omen: Legacy of Kain and The Operative: No One Lives Forever. :(
I'm also a bit curious about games like The Wheel of Time, Archangel, Drakan: Order of the Flame, Heavy Metal F.A.K.K. 2, maybe KISS: The Psycho Circus, but I don't know if they would still be fun for a first time player now or if they didn't age well. Heretic and Hexen should be here, too. And Max Payne, provided it works and is giftable across regions. ;)
rgnrk: The thing is, to me, it doesn't matter. For me it's not about playing the game to enjoy it with todays sensitivities, and more to preserve the game as it was made. So basically, I don't play most of the games that I buy.
(For instance, most Hitchcock movies haven't aged well at all. They will always be historically relevant, though. But it's not only about movies that important. Luckily most movies are preserved, so I have access to the ones I'm attached to, regardless of their objective quality. I wish that was the case with games too).
But if it's only to preserve the game, it would probaby be better off in a public museum or a library than in a commercial store like GOG or your private digital shelf (unless you plan on inviting people to your house to show them the history of videogames). ;)
Besides, I doubt the games I listed in the first part of the second paragraph are of a similar revelance as Hitchcock movies. And I'm not attached to them because I never got the chance to play them before. I'd be more eager to spend money on them if I could still play and enjoy them - and I would want to play them. But it's hard to tell which games would fall into this category and which ones wouldn't. I bought and tried all kinds of old games and some were still fun to me despite 'outdated' graphics and controls (e.g. Gothic, Tomb Raider, Blade of Darkness, Unreal), while others I gave up on after half an hour or so (e.g. Urban Chaos, Deathtrap Dungeon). For the latter games it would make no difference to me whether I'd own them or not. Of course, it's also pretty subjective, so I didn't mean to say GOG should only get these game if they have aged well in my opinion, but that I would only buy them at higher price points and be happy about the purchase if I could still see what's great about them.