Even though you didn't need it, I did the attunement quest prior to WOTLK. After the expansion came out I went with a group and downed Onyxia.
The group asked me to heal and I have to admit it was boooooooring. Through phase 1 and most of phase 2 I did almost nothing.
Finally I gave up and starting tossing Starfire at her. That was more fun :)
After she was down I won the roll for the Onyxia bag, but it was given to someone else. Oh well, maybe next time :(