I find that lighting (and view model distance) can make or break a game. [visually anyway]
I'll just post examples due to my time contraints, yes all are from Skyrim being as this is the largest modding area.
Dynamic LOD was a huge one for Skyrim when I found it and did not realise how much I needed it.
It would be cool and very cool looking in Wurm
ENB enbsite is always something that just makes lighting look amazing, now I know it's for DX9 (and DX10?) but I'm sure there has to be a OpenGL equivilant somewhere.
If these exist..... Wow, why have I not seen them yet, if not, I do hope either are possible