In the US, we almost never say "fair" to mean beauty, we always mean "just" or "equal."
What was fair about him taking a throne that legally should have been Stannis'? (not that I delight in the thoughts that brings) People reguarded Stannis as just, but he kind of missed the whole point of justice...
Well, the same justice that was in Robert's rebellion (that our honorable Eddard took part in)
Renly may dispense justice and mercy freely enough, and the people did love him... Yes. But .... I can't put my finger on it, but the people needed more than Renly. Could you see Renly preparing for winter? Sending armies to fight the Others? No. Why? I think he was in La-La-Land like Robert. "Others? What Others? You're always so serious and boring, kings hand of mine!"
Well, maybe he is not the perfect king, but imho he is a better king than Robb, Balon, Joffery or Stannis...
He wanted to help Ned (but Ned refused because of his stupid honor). He could be tough when it needed:
“If you cannot keep the king’s peace, Janos, perhaps the City Watch should be commanded by someone who can.”
And you have others things:
"The Hound snorted and tossed the broken antler into the crowd, where the commons began to punch and claw over the little bit of gold, until Lord Renly walked out among them and restored the peace."
"..strong yet generous, clever, just, diligent, loyal to my friends and terrible to my enemies, yet capable of forgiveness, patient-“
“-humble?” Catelyn supplied.
Renly laughed. “You must allow a king some flaws, my lady.”
We didn't really know Renly good enough, to judge him.. But he made a good impression on me... Whether he could be a good king or not, I wish he was still alive...