I think the hate comes from his constant "tinkering" with the original trilogy. Not only did he add distracting CGI, but he even changed the scenes aka: Han/Greedo cantina scene. He then went out of his way to ONLY release these new versions and not allow quality copies of the theatrical cuts. A lot of old school fans took that as Lucas saying "F*ck you, I'll do what I want and there's nothing you can do about it." That's where the hate really started. The prequals added gasoline to the fire because they were just so bad, especially the first two. Then when the DVD's came out he "tinkered" with them again to fit with the prequals. And of course when the Blu-Rays came out he changed them again. One final "F.U" to the fans before he sold out to Disney.
I made this same point earlier in the thread, but while I can see it as a reason to be annoyed with Lucas, I DON'T agree that it's a reason to hate him.