Everyone here has some great opinions and ideas on when someone is considered a "good golfer", so I'm not putting anyone else down by saying this, but in my opinion anyone who can consistently shoot in the 80s is a "good golfer". The reason I say this is the "average golfer" (IMO) shoots in the high 90s/low 100s. I read in Golf Digest either last year or 2 years ago, that only about 5% of golfers can consistently shoot in the 80s. While professionals and hard-chargers will probably be of the opinion that shooting scratch or in the 70s consistently is a "good golfer", I personally think those that are scratch or shoot in the 70s are a "great golfer". I base this off my personal experience and how few golfers can consistently shoot in the 70s or 80s. I have seen guys that consistently shoot 84-89 and while they are usually lacking in the short game and can shoot some errant shots from time to time, for the most part they have a consistent swing and make better than average contact on 80% or more of their approach shots. If you are doing that (scoring in the 80s), you are a "good golfer".
I will however have to agree with all the others that talked about having the proper golf etiquette, et al. I agree that someone who shoots "poorly" but has a great respect for the game, traditions and etiquette may not be considered a "good golfer", but I will not tell them that to their face. I want to keep people coming back and playing and working on getting better. While we may spend more time on the course when playing with those golfers that do shoot in the high 90s/low 100s, if they have the right attitude and outlook on the game, it is still a pleasure to play with them; and to build their confidence, I will tell them the things that I think they do right (within my scope of knowledge of course) and sugar coat the things that I think they do wrong. Hopefully that will inspire them to keep coming back.
But, like most of us, when you are playing poorly and want to quit and are cussing the game (in your head), it only takes one good shot on one hole for you to make the comment, "And THAT is why I keep coming back!"