And that's just it. Nothing was real. Nothing. There was no substance, like you said. There was no emotion and on the rare occasion that there was, it was so painfully and obviously forced that it felt as fake as the green screen effects we were watching. Without substance, there's nothing for the audience to connect with, which leaves the viewer empty, bored, confused, and minus whatever you paid to watch this crap. With nothing for the audience to really connect with, there's no purpose. Like Quickie said, everything looks fake and everything feels fake, and your brain notices it.
Someone asked before what the big deal was with the CGI Gophers in Crystal Skull. What you have to understand about film making is that things are kinda suppose to serve a purpose. This is one of the major flaws of Crystal Skull, in that too many things in that movie serve absolutely no purpose at all. When things lack purpose, they are noticed both consciously and sub-consciously. So when you put CGI Gophers in the movie for absolutely no reason other than to put them in there, it's a problem and it's a problem you tend to notice when it looks fake and feels fake.
It's no different regarding the difference between CGI Yoda & Muppet Yoda. CGI Yoda moves more, has more action scenes, and you get to see him bust loose with all his Force knowledge, but Muppet Yoda feels real. Despite the vast VAST difference in wisdom due to better dialog, Muppet Yoda is more alive, and you feel for him.