Niven's accurate treatment of scientific topics and his creative use of alternative worlds is outstanding too.
His well thought-out and highly-credible Ringworld (an artifact world along the lines of a modified Dyson sphere) and the barely credible, but just possible world of The Smoke Ring (a free-fall environment in the gaseous envelope surrounding the core of a gas giant - introduced in his 'The Integral Trees') are masterpieces of imagination. This adds to his reputation and readability, and makes him one of the most oustanding science fiction writers of all time.
This, with his treatment of possible alien species (I agree heartily with Suvrat above) means that he is, for me, a 'must-read' author.