And I think one major reason was something I just read in a fic, that made me stop reading the second I read it.
"They weren't just brothers..."
Those four (well five) words say it all.
Wincest means to me that the authors don't realize just how important a bond of brothers truly is. Being brothers or sisters or family in general (blood or otherwise) is to me one of 'the' most important bonds a person can have. And in wincest or any other incest like pairing, that bond is treated like it's meaningless. It's twisted into something sexual and to me that diminishes the bond that's there.
It takes something that's core, deep, probably the greatest bond any two or more poeple can have and restricts it to something purely physical.
I hate wincest because to me, it wrecks the bond between the Winchesters and lessens it.
And no, it's not because it's slash, I feel the same way about Dawn/Spike, or Buffy/Giles or Buffy/Xander. (and those don't even have a blood relation.)
So yeah, I hate wincest, and I really truly wish that it didn't keep sneaking into fics that claim to be gen.