@Dragonbird: Let's compare use cases of Steam and GOG right now:
1. Steam
User opens the client, and installs some game in it. Further management of udpates happens through the client (user doesn't need to manually figure out what to download and install to update the game).
Good - convenience of automatic management of updates. Incremental updates means less time to update the game.
Bad - DRM. No easy way to back up the game or the updates themselves (as standalone deltas).
2. GOG
User has no client, besides downloader helper which right now has an open source community alternative:
https://github.com/Sude-/lgogdownloader plus of course one can download games from the browser. In case of any updates user has to go to GOG forums to check what exactly is updated, download the whole thing, and reinstall the game.
Good - No DRM. User can back up any current version and reinstall later again.
Bad - Manual process. No incremental updating means the whole game installer / package has to be downloaded in full increasing download time.
So how do you envision a better service? I'd say combine what's good in both. I.e.:
3. Hypothetical new GOG
There is an updater client which manages installation (optionally) and incremental updates. In order to be DRM free, it also gives an option to save the full package as well as update deltas (so if users saved only the first package, applying the deltas would bring them to the final version without the service needed).
Good - convenience and no DRM.
The only problem with this approach is that the client has to be open source in order to improve trust. Having any closed source code installing stuff in your system is an abysmally bad idea.
For example, Desura has an open source updater client:
https://github.com/lodle/Desurium