I hadn't been following either Nightwish or Kamelot for a while and didn't know Kamelot had a new singer.
I also "outgrew" Nightwish some time at the end of "Tarja's period" and never got into them with then new singer Anette Olzon. I listened to several songs from their 2012 album right before the concert, and thought it was worth going - they seemed to reinvent themselves, not necessarily in a style that suits my taste 100%, but this was promising to be an interesting concert, definitely not boring.
Kamelot was opening for Nightwish, and since they are more up my alley than Nightwish, I was hoping for a bit more time. They were great!
What happened with Nightwish's show perhaps is better explained in this article:
In short, singer Anette Olzon was taken to the hospital right before the show, and Kamelot backing singers ("Kamelot girls") did an amazing job filling for her. I don't think anybody left the Ogden theater after the announcement that this will be partially "mass karaoke" concert. I regretted not being familiar with songs, normally I know every single song at the concert I go to, but not this time.
The article also has a couple of videos that I thought did a decent job showing the atmosphere - crowd was super-supportive, and Elize and Allissa did an outstanding job!