A bit off-topic but I am really in favour of the idea of replacing Jar Jar in TPM with C3PO :P
But more on-topic, I think that the lack of physical interaction made it harder to relate to the droids and aliens in the PT... instead of acting against something physical, the aliens and droids were more often added in later and you can really tell when the actors are just staring in the general but not wuite direction of the characters that are going to be placed in later.