When browsing these forums or the Nexus the most mod I see are altering gameplay mechanics, enhancing textures, are ENB-Mods, single weapons or armors, but only very, very few mods that add real content to the game. I really miss good quest mods! I know and really like the big mods like Wyrmtooth and Falskaar, but there should be plenty of good mods that add small questlines to the main game.

Is there a site like Skyrim GEMS (http://skyrimgems.com/) for Quest mods? I'd really like to try some of them but I don't want to test every single one if it's well done, lore-friendly, relatively conflict-free and so on.

I'm not aware of a site like that. But here is a recent thread asking for recommendations for quest mods, if it's any help: http://www.gamesas.com/topic/1475642-helpwhich-quest-mods/

I think part of the problem why people have avoided quest mods is the whole install then can't uninstall thing. So basically those mods need to be optimised very well such that people can just drop them in and keep it forever, which means high quality, which means lots of time, effort, etc etc. I think recently there have been a couple of good small quest mods but nothing like Falskaar or Wyrmtooth.