will the downloader ever add launcher functionality? for example be able to start any game installed in the gog.com folder. similar to steam. I don't like have to use shortcuts on desktop/taskbar
I think not, the downloader is strictly only for downloads and notifications and many people do not like your idea.
Granted, some sort of launcher may be neat, but if you buy (digital) games from different sources and whoops you have half a dozen mandatory launcher/clients gobbling up resources on your computer. Personally I don't really like the official GOG downloader and I'm using the web interface (more reliable) for downloads, but this is due technical reasons. I'm glad having choices on downloading games here on GOG.com.
Technical question for a blue: Does the GOG downloader internally use 'rsync' as the mechanism for downloading the game files etc.?
AFAIK the GOG Downloader uses a custom download method with checksummed chunks for all stuff. The size of the chunks are not as fine-grained as rsync's, but optimized for paralell downloading and saturating the available bandwith.