voice level. Was it appropriate?
pronunciation of words: could you understand the words? I get so irritated at choirs that sing like they had oatmeal in their mouths.
level of emotion: was it appropriate..compare Madonna on one end, and a Vespers choir on the other.
I would also critique the musical accompanyment.
Now the words: use words that describe these things. Dull, exciting, powerful, quiet, full of energy, off key, perfect timing, serious, crafty, seasoned, ...
Bobpursley is right. There's a whole range of words to be used.
Last December, I attended a choir concert composed of two elementary school choirs, one choir from the high school, and several different divisions of the choir from the host middle school. I couldn't understand why the middle school director divided up that choir into so many pieces. Perhaps it had to do with the natural reticence of middle school students to perform in much of anything except sports! But the elementary and high school school choirs gave spirited and varied performances (and by "varied" I mean varied in emotion, tone, volume, etc.), whereas I couldn't see any difference among all those different middle school groups, all of which were too quiet, too emotionless, etc. It was long, dragged-out, and monotonous. Only the elementary and high school choirs' performances added spice to the whole concert!
Use any of the words Bobpursley gave you that fit. Also, don't be afraid to go into a good thesaurus to find variety in your own wording: