The basics which they seam to forget! I used be one (please no boos!) and the clients used to be really nice and say I was good but I couldn't see what I was doing any different than the others but now I'm house hunting and out of the industry I can really see it and get so frustrated!
Be honest, be punctual, keep in touch and do what you say your going to do!!!!!!!!!!
I want them to be straight with me, do what they say they will and be on top of the paperwork so you don't get stupid delays.
Mine deserved his commission when I sold my old house - he told me exactly what to do to make it look more saleable, produced some good specs that made it look nice to get the buyers in, did lots of accompanied viewings and was generally efficient re feedback etc.
The wide ones are the worst, when they make stuff up like phantom offers.
I've just sold a flat using one who was brilliant at keeping everyone onside (very dozy FTB that I would probably have dumped if it hadn't been for him - and ended up wasting more time finding another buyer). He called just enough, did a lot of running round when FTB decided to go on holiday without setting a lot of things up, put himself out a lot once the offer had been made in order to keep everything (just about) ticking along.