Profile pages would rock, just to show off the neat games I've bought.
Such profile pages could also contain a game wishlist, so you know what to buy for your friends.

Many games on GoG got multilangage, support it in the website and installer will be great.

1. Another vote for Personal Wishlists.
2. Add the game's title to the "Basic Details and Overview" section (as text) so it's easy to copy and paste if I want to search for it or whatever.

1. personal wishlist with games you would buy.
2. a complete overhaul of the current wishlist. It started as a great feature that I checked regularly. Now I dislike and don't intend to use it anymore.
a) too many joke entrys like (at best) mediocre games for your 6 year old kid
b) a lot of double entrys
c) entrys for other systems (Amiga) which are as realistic as the zombieneighbor next door
d) no overview at all
Either come up with a different system or someone has to moderate it.
3. upcoming games. Too little, too less. Can you expand it to additional sections of "we're in talks for" and "we're trying to get" - of course there's no promise on both sections that you'll succeed.
4. More options for the board. An ignore-function for example.

It would be cool if you could activate or register games that you already own hardcopies of on GOG. For example, I already own Atomic Duke3D on disc and it would be nice to have a backup online and have access to the bonus goodies. Though I do realize that the bonus goodies are probably an incentive to buy from GOG, which is cool :) I'm not even sure how you could keep track of this, as many of the older games don't have a key or unique serial number of any sort to verify that someone owns it.

I don't see that happening due to the DRM-free nature of GOG games. With Steam (which is probably where you're getting the feature request from), they can yank a game from you at anytime and they know you will no longer have it.
If a game is pulled from your GOG shelf (I don't see why this would happen), you still have the DRM-free downloaded copy to play when you wish.
The advantage to the publishers of having users register their game keys with Steam is that they are then locked to that account and other future owners of the hard copy cannot use them to play online. If the first owner didn't register the code, the game could be sold and without any issue for the buyer. If the key stays unregistered, this process could continue for quite a while.
In short, I don't see GOG doing this as their product is fundamentally different from original retail products. Just my 2 cents.

+1
Almost every game on GOG has a spanish version available, so, why don't you guys add languages pack to the games when available?

+1
I would like to see a feature such as this. I tend to download the games I've bought, burn them, and (usually) forget about downloading them again.
A feature that would indicate any changes would be very handy. Date added/modified would be cool, or installer version numers visible?

Gexecuter mentioned in a separate thread an auto-shutdown option for the GOG downloader would be great. Yes yes!

I want this aswell. Most important would for me would be a way to check if I already have the latest setup-file.
Best case would be if GOG would remember the version I (last) downladed and displays a "NEW" next to the file.

This isn't the game wishlist thread, it's a wishlist for the site and service itself. Go here for the game wishlist.

I'd like to be able to sort the game catalog by publishing year.

the muse wolfenstine games or the id wolfenstine games ?
i would like to have gog.com versions of the shareware demos made so that people can play before buying some of the more obscure titles the bulk of the people don't know about some of the early apogee games that lead the way to duke 3d

Just a little e-mail warning or such that notifies you that new content is available for your game (whether it be a new soundtrack or updated installer) would be great.