Handman said:
No Jar-Jar.
I think that's too simplistic an answer. I don't the the problem with Jar-Jar is his existence but more the way the character is presented. The voice, the cartoonish look and slap-stick, the unfortunate and unmistakable flavour of Stepin Fetchit.
A light relief alien character which acts as a plot bridging device would have been fine but the tone of the character wasn't Star Wars.
If you look at aliens like Yoda or Chewie or even Jabba they have a realistic feel to them despite their nature. There's a physicality to them and they feel like people.
Threepio is fussy and gets on the heroes nerves at times but once again he feels like a real person you might meet and could care about.
Artoo has the nearest amount of physical humour to Jar-Jar as a main character (he gets spat out of things, thrown across rooms by electrical shocks etc) but there is still enough 'humanity' in the performances drawn from the machines, Kenny Baker and the sound design for the audience to connect to the person that abstract shape represents.
Jar-Jar is aptly named because the way he is presented is iike dropping Roger Rabbit into an official James Bond picture. He just doesn't fit, he Jars.
If the same care had been taken as was taken with the OT characters he could have worked.
He is a touchstone as to what went wrong with the PT. It's just too cartoonish and lacks enough human spirit.