Lucas has a vision, and his purpose in making the films was to put that vision on film. Empire, although the original story is written by him, I have heard is not his favourite.
My impression is that he is not jealous, but rather, he simply doesn't like the film because it doesn't come close enough to his vision.
I recently watched all 6 films in order (1-6, not 4-6, 1-3) and I found that there was a definite sound... a definite look/feel/tone to his films.
Empire, however, stuck out from the rest.
For one, the characters established in ANH are expanded upon, but it is done so in such a way that it almost mocks the characters.
Han and Chewie's relationship and the Falcon never working the entire film. Yes it's called a bucket of bolts in ANH, but it works. It's been taken care of. In Empire, it seems like they've let it go and it's almost a lost cause.
Irvin Kershner has said before that he wanted to do a film that had humor in it. Look at all the other star wars, and humor is appropriate.... but that George said he couldn't really have any slap stick humor, which isn't appropriate.
So instead, we get situational comedy and inter-personal comedy... neither of which were really appropriate to the extent we see them....
Most of 3P0's lines were adlibbed and yes, almost all of them were perfect for the film and did show his character, but I think that they took a bit too much creativity in it and I think that's what George didn't like.
Also, to exemplify the amount that the writers didn't quite get the characters, one need but look at one of the most famous lines in the film.
Leia: "I love you."
Han: "I know."
In the original script, he says "I love You," but as Harrison Ford so keenly points out, Han would NEVER say that...
And if someone who scripted/wrote the film is neive enough to think that Han would say that, then they truly don't understand the character...
Then listen to the score. To many fans, Empire is their favourite star wars score and I have to say, it is a GREAT score... But it feels entirely different. The fact that almost the entire film is scored, and the music that is unused, if you put it back in the film and compare it, you can see how Williams really felt about the film and where his mind was and it's not very "star wars"-ey.
Great score, but in a lot of cases, it is a different feel than a normal star wars film.
I mean, the falcon was written in the original story to not work. But when something doesn't work, there is more than one way to do it.
Han: Ok, chewie hit it!
The hyperdrive whirls up then powers down::
Chewie: Roar!
Han: Quick Chewie, check the (insert techno-babble here). Princess, keep an eye on this guage.
Leia: What? I don't know anything about--
Han: Well, your highness, want to be captured by the Empire? You forgot how fun it was last time. ... Watch the gauge.
Instead we get:
Han: Ok, Chewie hit it.
::engine whirls up and powers down::
::Leia looks at Chewie, then they booth look at Han::
They do the same thing, but one is more strict and tense, while the other is comical and to an extent, inappropriate. They are being chased by Imperial Star Cruisers after all.
These are just my opinion. When I watched all the films, it stuck out the most in my mind as being the most different of all the films. It is a lot of people's favorites and I can see why. It does a lot and has a great story to tell... but I personally think that's why George was displeased with it.