I can see perfectly why caddies are hit and miss because I've had both a really good one, and a poor one.
I think a good caddie knows to enhance your experience instead of becoming an attraction or sideshow to the round.
Here are good attibutes IMO:
First off as a general rule I think any good caddie understands the "speak when spoken to" mantra. Its not about being a prick as the player, its just that some folks like chattyness and others don't, so best to be silent by default because that usually doens't offend anyone.
Second, its great when a caddie hustles and this was certainly the case in my situation. He was johnny on the spot with everything we needed including clubs, yardage, ball searches, water bottles, etc. He was 2 bagging and I don't remember having to wait even once.
Third - Have a good positive attitude. People can pick up on this even with all the non-verbal clues. The guy seemed geniunily engaged in our round and we had a few fun chats along the way, when we initiated the conversation.
Fourth - When asked for advice, be sure to know what your talking about. If a putt ends up breaking 12 inches in the opposite direction than you said, you're not going to have a happy camper on your side.
When these things happen I will glady fork over the dough and really appreciate the service, because it enhanced my experience...