It's all about playing with bottles, having the same facial expression over and over again and loving some crazy vibratos.
j/k
for me, what describes an "Avex diva" (weird term) is a female artist under Avex with a LONG and lasting career with great success. They happen to have influenced Japanese pop culture to a certain extent and are leaders in terms of fashion, beauty styles and musical success, with long, lavish tours and PVs, an army of fans and of haters.
Only Ayu, Kuu and Amuro qualify as current Avex divas. Notable mentions from the past would be hitomi and Ami Suzuki, too.