Alot of good and reasonable things have already been said. Imo a good instructor needs to have 3 things:
- Diving technique and experience. He needs to be a good and relaxed diver. Fortunately it's not very hard to become one. You just need to dive ALOT and in different circumstances (good vis, bad vis, current, cold, tropic, boat dives, shore dives, quaries, inland lakes, ice dives, deco dives, wreck dives, etc, etc). Next to that you need to be able have the luck to watch very good divers dive. From all walks of life and different organisations, so that you learn there is not one truth, and learn to recognise the pitfalls in every education. So basically if you surround yourself with the right people and just love to dive whenever you can no matter where you'll just need time to get this.
- Didactic skills. You need to be a teacher. Some have this naturally some get this through professional experience (being teachers, trainers, in leadership positions), some have to work very hard to get this and some will never do. If you can't convey your diving experience and love for the sport why are you teaching? If you don't have the patience or the vocabulary skills to convey a message or a technique how will you become an instructor? Finally you need to love working with new divers. Love watching them clear their first mask after numerous attempts, love them taking their first underwater breath and love it when they finally get the 'buyancy trick'.
- Personality. This means both how you function under stress, how you make decisions, how calm you can be, etc.
The above comes from a voluntary organisation point of view so it might differ with the commercial organisations.
I'm no instructor and won't be in a long time , but I have dived a bit with new divers. And it's always a pleasure... just small tricks... letting someone use your fins with springstraps to show how easy shore entries can be with a small equipment adjustment. Or doing a propper weighing and let the person experience the effect this has on his trim problems or explaining and showing how underwater navigation can be done also in bad vis with a combo of compass for direction, underwater points, depth meter and time. It's not instructing but it's just passing on some basic things that you've learned from others.
That being said: I've been led by the hand the first 50 or so dives by a variety of good instructors (I still dive with them on weekly basis):
-W a 1500+ dive instructor who's interests lay in north sea wreck diving and the bigger animal live in the tropics (hammerheads)
-G a 800 + advanced instructor who's taken me along for some really nice dives... his insights into dive medicin have also been quite good.
-G a 10000+ dive instructor who has been diving since early 60's did a tour on the calypso with Cousteau, was a professional diver (safety diver)... I drink mostly beers with this guy but we share the shame passion for wrecks.
-E a 1000+ instructor who used to be a safety instructor in coal mines and has seen it all... if we're talking high stress quick decision matters he's the man.
And many more instructors and non instructors...
Anyway my cup of salt