Way too many pet-peeves to mention. I could go on forever with maybe hundreds. One that sticks out is the guy that spends 10 minutes lining up his quadruple bogie putt. Or the high-strung idiot that expects par golf out of himself, but has a 16 handicap and can't handle the fact that it's a game and bad holes are out there.
As far as a good playing partner. I believe it's the individual who is :

1) Humble- knows golf is difficult and can laugh at himself.

2) Goes out for the asthetics the surroundings hold.

3) Gets as much enjoyment out of his friends as the game itself.

I think all the other physical aspects such as etiquette and level of play are secondary to the person him or herself. I would rather spend my free time with an enjoyable person far and beyond golf's "rules."
Unfortunately, it can be like finding a needle in a haystack. These people are hard to find. Nice thread.