Letting us read the manual before buying the game.

Are all the games patched to their most recent version?
I like the idea of a launcher, but not to have it be required.
It would be sweet to have a mod option in the launcher, ie, in Torment be able to activate/deactivate the halloween themed character sprites, in Baldur's Gate be able to enable/disable the fan created "missing items and quests" etc.
Does anyone understand or have I completely not made sense?

I heartily second this! Also indicated if it can run windowed mode or on a widescreen monitor....

Having demos so we could try the game before buying.

++ DVD covers and disc labels

OS X support - my main machine is a MBP, and while I can fire up a VM or Bootcamp I'd rather just run older games without that extra step.
Just offering the original DOS downloads so I could easily toss them into Boxer would be great, though that doesn't really solve the win9x titles. :/
http://boxerapp.com/

blablab

I'd really like to see the game wishlist cleaned up. It's severely broken due to the fact that many games are added to it several times.
Take Aliens vs Predator 2 for example, it is listed in the games wishlist under the following names:
Alien Versus Predator 2 (126 votes)
Aliens Vs Predator 2 (363 votes)
Aliens Vs Predator 2: Primal Hunt (223 votes)
Consolidating these into a single game vote and eliminating double votes from users (a user could have voted for all three, severely falsifying the result if just blunt addition would be used) and make people think before adding games to the wishlist.
I.e. you could implement a "did you mean <game title>" function to keep people from adding a game several times and f**k up the voting mechanism. Thereby you will have a vote database that more accurately reflects peoples wishes.
Oh and yes, the mentioned things for the games in their respective product description pages would be nice:
- multiplayer capabilites
- input device support (joysticks!)
- supported resolutions
- widescreen support
- dosbox, scummvm, native etc
- links to support forum
- demo download if available
- links to must haves and community sites (FreeSpaceOpen Installer for Freespace 2 for example)
Oh, and slap the people who mistake this thread for a game wishlist. ;)
j/k...

Here is my features wishlist...
-AlertPay as a payment option (if it is possible).
-DVD covers for the games. For the collectors (including myself).
-More free games to get more people interested in the site. :)
This is for now. ;)

It could be useful to know if a particular game can run on Linux systems! I mean I know there are some games running with DOSBox (Wine and Cedega also?), so I would be nice to know if someone had the chance to test it (by putting a ranking system in the game page).
I'm a Linux user and thank to dosbox I could play some old games without switching the OS =D
What do you think? :D

Correct me if I'm wrong, but doesn't Cedega enable virtually any Windows game to run on Linux? If so, Cedega emulates Windows, which runs DOSBox which runs game. A bit complicated, but since hardware performance is not an issue with today's rigs, I'd find that to be a viable solution.
Of course, running games natively would be much more elegant, but it's always a question of resources and demand. I am a Linux user myself, but over the years I have found it easier to set myself up a second machine with windows exclusively for gaming.
If the Linux userbase of GoG is big enough, then go for it, CDP, but if there are only a few people out there, then it would be hard to justify the cost of the development and testing time.
Also, you'd probably run into some serious issues with version conflicts etc with all the different distros out there (foo.lib might be another version on Debian than it is on Red Hat etc). The Linux and OSX guys would really have to make this worth CDP's while...

Also in for a gifting feature for both members and non-members of GOG. I'm sure this would stimulate the sales of games a lot.

I belive that it would be very nice if you would update the wishlist, merging votes for entries about the same game (but writen in a different way) and update it to recognize and eliminate games that are released by GOG already. Because, if you look for it, you can still find Screamer there for example.

Real, physical gift cards sold in Walgreens and stuff.

GOG should make a deal with ULPC (eee, wind, one etc.) manufactorers (one free game or something) since these small laptops are ideal for old games - with small monitor (7"-10") old games looks better than with >20" monitors. Millions of ULPC's has already been sold and the growth in unit sales increases year after year so GOG would probably get lots and lots of new customers.