I think for me when it changed from "Star Wars can be this" to "Star Wars must be this". Lucas' introduction of both the special editions and his type of prequel films contributed to an already growing problem in the EU of boxing stories and creativity to the point where things in Star Wars must fit into specific guidelines.
If Lucas had introduced the Special Editions and as what if concepts or even as an amusing film experiments (while still maintaining the originals in decent quality) I don't there would be such a divisive split as to what's real and what's not. Yes debates about which was better sure, but certainly not the almost existential arguments which ensue about what is "true."
The whole thing with Greedo and Han took away the relative nature of how you interpret Han's character into a more simplistic back and forth about what's absolutely true to his character. Beforehand you could say that Han doesn't care at all, that may not have wanted to do it but had to, that he and Greedo had a personal backstory hence it was out of emotion than reason, all sorts of stuff. Now its been broken down to "Han's a good guy, he wouldn't do that" and "Han's a scoundrel rogue, of course he would" Its a more narrow definition to something that once felt ambiguous.
Problems exist with the EU as well. Because of the sheer influx of stories based on a business model rather than what's ideal for tone and character, a lot was lost. Star Wars isn't a mystery anymore. Its not built on reader/viewer imagination and enjoyment as much as it used to be; its been distilled into the very act of choosing personal canon based on specific stories. Going down the EU feels more like checking a grocery list than being exposed to wondrous other worlds and tales.
Its ironic because the original Star Wars was hailed by some in the press as working because it was loose and ambiguous enough to be able to project your own meaning onto the movie. It worked for everybody. That's just not the case anymore.