Like I said, just my opinion. But I tried most of the really good mods. And I would be lieing if I say they don't improve the game. But even all of those mods, still don't change what Skyrim is at it's core.
I will say this again. The main difference is, that you don't need really mods to enjoy The Witcher or Gothic 2. Those games can stand on their own. Skyrim can't. And it shows. Even with mods. You would probably need more than just one mod to get the game in a position where it is really enjoyable. And what they most of the time do, for me, is to remind me how sloppy Bethesda was in their work. And that's what I mean with the painting. There is no doubt that Skyrim is a "finished" product. And not an empty canvas, that fits more to something like Minecraft.