It all suggests that he lacked confidence in the new characters he'd created - how many of them are iconic like Darth Vader, Luke Skywalker, Han Solo, Chewbacca, Yoda etc? There seemed to be no effort to create decent new characters, which is strange because from what I hear about the books, there were plenty in there, and even the animated Clone Wars series had more interesting characters.
As for the Jabba scene, the fact the Greedo scene had already made it redundant, plus the problems associated with replacing a human with a giant slug, should have told Lucas not to include it at all, but he seemed to have completely lost his sense of judgement / quality control.
It's also sometimes a sign the writer wants to say, look at me and how clever I am. I've weaved a narrative that has all these characters interacting over time and influencing each others lives so well. Fine in one film as you keep it simple, fine if you plan for it (like in Babylon 5, i.e. seemingly throwaway comment in S1E2 say has massive repercussions in S3 and S4) but not if you didn't plan for it over many films and certainly not if you include wonderful abilities for said known characters that they never display again.
There were new characters in the prequels he could have run with successfully but he ruined it by spending more time on the CGI then anything else, introduced bloody Jar Jar and went with the most piss poor romance ever. I remember laughing in the cinema at how bad it was and how forced.
On top of that the relationship between Obi Wan and Anakin was awful, we kept having to be told how great it was and how much fun they had whilst they gave every impression of despising each other when interacting. Or some of the outright stupidity the characters indulge in that make no sense at all for supposed wise Jedi knights.
RLM's reviews say it better than I can, so for anyone reading this who hasn't seen them - watch them!
For the last point, yes again it was a clear indicator that Lucas had lost focus and was shoving back in a scene that was unnecessary just for the sake of CGI. Prequels were just the next one to suffer a few years later. The excuse that it was added as it was only dropped in the first place for lack of CGI is not sufficient. The original survived without it so don't feck around Lucas and add in things that are not needed.
