1. Nintendo 64
2. Super Nintendo
3. Sega Dreamcast
:D

All an emulator requires is resources and time. Given each game would be packaged or wrapped just like our DOSBOX games it's highly likely that GOG will make sure any emulator is as close to 100% as possible. This is easier to do when you have a legal agreement with the console maker. Also don't forget how close a Dreamcast was to a PC architectually.

Fair enough. Likely they would have to cooperate with the companies to make something working.
Steam isn't dedicated one way or the other. Anyways, it's not a question about can or can't, but approachability and demands. GoG promises to offer DRM free games to any location in the world, with equal prices everywhere. Steam allows the distributor to tack on their own DRM in addition to Steamworks, different prices in various regions, and regional lockouts. The latter may look a lot more attractive to the publisher than the former, since they can make their own demands to have it's to be distributed.

Well i agree with the answers from Shapedragon, beside that i would like to add that the success of this site surely didnt pass unseen by other on-line sellers Valve-Stem as first, you can check lately they are adding pc ports of old SEGA consolle games in example, and the LucasArts Adventure pack is something quite important to understand how the retro gaming is now considered sellable.
I bet they come here to check the wishlist and try to get those titles on Steam, ofc with DRM etc.
Beside the fact i find opinable what interest could have anyone in playing a beat'em up like altered beast in 2010 on a pc without a digital joystick, its a clear signal that Stem heads are considering the retro-gaming something that actually can have more weight than planned.
Steam has many advantages on GOG to get whatever they want as "old games" cause they use copy protections, they sell games at higher prices, and they have contracts with all the majors, so if Bioware will relase a DD version of Planetscape Torment one day, we will see it on Steam at 15/20 € with all the restrictions Steam has.
GOG surely knows that better than me, so thats the reason why they are exploring other possibilities.
Thats the reason why i personally think if GOG could get the rights to distribute other platform games it could be a big plus, in example getting Cinemaware catalog (AMIGA orignal roms ofc ) or ultima orignal roms, zelda, FF or whatever epic saga/game... it will result in a worldwide bump in GOG relevance as DD games site, as every computer magazine in the world will write something positive about it surely.
I say GOG should go for the game, not the platform, dont think playing kick off or speedball with keyboard could be so fun, strategy, rpgs , simulations games, point click adventures are games that can be played now without loosing the feeling that original had.

Atari ST (I know it's not on the list, but I'd really like the best version of Captain Blood and that was on the ST :) )
Amiga
C64
As long as GOG continue supplying a great DRM-free catalogue of good old PC games, then I don't mind seeing other formats of good old games DRM-free as well. But remember that the PC should be the priority.

1. Amiga
2. C64
I also agree that this would be a nice BONUS if it won't impact the release of PC-games in a negative way.

1-Commodore 64
2-Amiga
3-Arcade

Wow. I had NOT expected this at all. But maybe I should have.
Anyway, I have only one old platform that I actually have fond memories of (not because I didn't like other consoles etc., but because it's the only one I owned) and that is the
1-SNES
2-
3-

1. Nintendo 64.
2. Super Nintendo.
3. Sega Genesis.

1- SNES
2- Dreamcast
3- NeoGeo
SNES for the classic RPG's (Chrono Trigger, Final Fantasy 6) and Mario games if you can get them.
Neo-Geo for the Metal Slug series and some other classic games.
Dreamcast almost exclusively for Marvel vs. Capcom 2. :)
Although Street Fighter III 3rd Strike might be good too. And Resident Evil: Code Veronica.

1.) Sega DreamCast
2.) Nintendo 64
3.) Arcade
The Sega DreamCast deserves to be on the top of the list since it has many tripple-A titles (Sonic Adventure, Shenmue, Skies of Arcadia, etc.), not to mention the hardest to emulate.

That's kind of my point. If Dreamcast games were sold on GOG pre-packaged in a custom emulator package that is almost 100% complete then I for one would be willing to throw down $10 for one of the platform's premium titles. I think most people would be happy to do the same for a near perfect port of Soul Calibur, Skies of Arcadia or Shenmue.
Thats Dreamcast of course. Amiga games would have to be exceptionally good even to warrant $3.99 and C64 even less, Maybe 99cent. Publisher packages would probably be the best way to go as I can see a lot of people happy to pay $9.99 for the complete Bitmap Brothers Amiga collection.

1. Commodore 64
2. Amiga
3. Arcade

1. It is EXTREMELY sad that ZX Spectrum is not an option :(
2. Amiga
3. Commodore 64
4. Another very sad thing is: why there are no really old PC games, like from the beginning of the eighties? Even in a special "untested, no guarantee" category? I would love to have all my old PC games @ one place - the one I am supporting financially on a regular basis. May I ask what my fellow users think about that idea?