1. Video's from actual game play or
2. Opening cinematic's
3. Friend's list

i think someone posted this earlier, but a "gift" option would be cool.

okay, so everything's already been mentioned that I had on my mind, so i'll just reiterate:
1) Multiplayer Support Groups (aka: ladders/clans/etc even)
2) "I Can't Wait for Payday" Wishlists
3) Videos of essentially any gameplay/cinematics/etc centrally on the website would be great, since I don't need another reason to spend MORE time on youtube.
And that pretty much sums it up for now. Hope to see some of these things in the not-so-distant future!

I would like to see support for Download Managers. I am currently using Free Download Manager - which has been running beautifully for years. Now I cannot use this, and Firefox has the worst download resumption ever, and I have already lost 380MB to the internet gods.
Anyone using download managers successfully, please let me know...

First can I say that this could be a breakthru site if it expands the catalogue. Really support you guys and I'll spread the word as far as I can.
Wish list?
Since so many of the games are old, the resolution is 800 x 600 at best, often as low as 600 x 400 and this makes play unacceptable on modern monitors. Either you have a boxy full-screen, or as people here are requesting, a window which is too small to use effectively. (point and click becomes hit and hope and fps has you shooting your desktop icons more often than enemies).
It is not likely but if it were possible then I would suggest that as well as making all games XP and Vista compatible, they are also tweaked to make them run in modern resolutions including widescreen. At the very least they should be set to run in 1024 x 768 as a minimum.
Look at the Widescreen tweak for the Baldurs Gate Planescape series developed by the Gibberlings group. It is simple and makes these old games playable once more in 1680 x 1058 glory (even higher).
I'd be against download managers personally, but that is my choice.
And a final word of warning. If this site does become successful, I can see it being reproduced with greater funding and advertising behind it by the likes of Steam and I-Tunes.

Both of these would be awesome.
Also, someone mentioned being able to add games to wishlists. I rather like that idea.

Creative ALchemy Detection, say a game is installed on Windows Vista that uses DirectSound3D (or DirectSound). The installer should, if detected, add a entry to the local ALchemy database (a ini file) and recommend the user to set ALchemy to the game or copy the files (in ALchemy directory) after adding the entry.

I'd like to see a Download all (downloads per game) rather than clicking the individual files.

To tell you guys the truth I'm too damn lazy to flip through the 11 other pages in this thread. So I'll just blurt my suggestion out and hope I don't get hanged for babbling about something obviously already discussed :P
I would like to see a feature to track 'coming soon' game titles. Like, you can browse the 'coming soon' list and then you see a nice game you certainly decide to buy - then you could just click a small link or button and subscribe for an email notification of when that came is available for purchasing on GoG.
Thanks in advance and thanks for reading this, everyone. :)
Go GoG!

+1 for demos
payment via paysafecard (http://www.paysafecard.com)
+1 for Wine compatible / Linux games :-D

Demos will be good for me too..

I'd like to have those fancy boxartsie pictures in high resolution as a downloadable game extra (just like wallpapers and such). :p
For those who don't understand what I'm talking about, I want high resolution versions of these:
http://www.gog.com/upload/images/2008/07/6b6f1b2978bf3d566626a316f773afb0b59542f5_bb.jpg

While GOG doesn't offer it directly (yet?), you should, hopefully, be able to find many of the covers at www.cdcovers.cc.
If you're looking for that cover specifically, for Descent 1 and 2, they don't have it. The "cover" for the Descent 1 and 2 pack is actually just a cardboard slip case, not actual CD jewel cases.
EDIT: Corrected the link and adding another feature wish:
When posting a link using the URL tag, have it "auto-correct" when posted. I left off the http:// part when I put it in, and it posted without that, so the link just linked to:
http://www.gog.com/en/forum/general/gog_039_s_features_wish_list/www.cdcovers.cc
Instead of the intended destination.

Possibly. But it could be just as likely that the publisher (if they actually created the cover images) may have disposed of the master hard copy and just have a digital copy. And for older games, it could have been lost or stored in a file format that current software doesn't actually support. Or maybe it is just gone, if they no longer publish the game in question.
Not saying I don't think they shouldn't offer it if they could, just saying that they might not be able to. I know a number of people, as well as myself, would enjoy seeing provided cover art (with or without GOG branding) as well as CD labels.