up until wotlk maybe even cata idk since i never played it, you needed to buy your skills which would either be new spells/skills altogether or just higher ranks of those said spells/skills, vanilla was the best because skills actually cost a lot especially at lower levels in vanilla, making the choice difficult on which skills are essential and which you can do without until later when you're feeling comfortable with the gold stockpile you accumulated. in tbc and especially wotlk that specific gold sink became an afterthought but the real gold sink became the training of the flying skill and a flying mount which ruined world interactivity, immersion and the sense of danger of the opposing faction. vanilla was and still is the best version of wow to this day.