Just gonna say Guild Levels and "Cash Flow"...still the worst as far as damage to wow has been
Imagine your a newbie "OH BOY A GUILD INVITE HOW SWELL" congrats your in a sweatshop guild no one cares who you are, how to help you play...etc. smaller guilds ended up struggling because they can't level as fast, and droves of new players left because these sweatshop guilds were their first experience in wow...
For the vets..those new players never came back...servers stopped getting new players because well "hey man don't join people just use us as cattle"..suddenly servers started going down hill cause half the new players either left or became the crappy "Just carry me" people we have today.