Let me start this by saying that Roberta Flack has paid her dues and given the world a wonderful gift with her voice and songs. She has an amazing history and many accomplishments. That said, that does not give her an excuse to be an unpleasant diva on stage.
The concert was off to a bad start from the get go. She was performing her first song and she kept tugging on her hair to cover her face. After the song was over, she said that she didn't like the camera shooting her from the left side and that they needed to stop. People kind of laughed at first because we thought it was a joke but she said that she was serious and that the camera needed to be changed "now". For the rest of the concert, the camera was a far away shot of her so you couldn't even see her on the big screens.
At one point during the show, she kept on staring at someone in the audience and she was sort of mumbling her lyrics as if she was too distracted to sing. When the song was over she pointed at the person and asked who they were. Apparently they were press and she told them to stop taking her picture because it was distracting. Then she called security and they were escorted out of the venue. This totally ruined the mood in the audience. It was awkward to say the least.
During the show she, again, starting complaining because there were some women who were cheering during her one of her monologues. She asked what was going on back there and some girl screamed "I love you Roberta" and she didn't even acknowledge it and just went in to her next song.
The other issue I had was that she was singing so much lower than her backup singers so it made it difficult to get in to the songs. All in all, she seemed very unhappy to be performing and that feeling rubbed off on the audience.