Really? I thought it was pretty simple. You realise that games exist that allow you to pay to get an advantage that non-paying players can't get, right? Most well known is probably the armour-piercing gold ammo from World of Tanks - a player who bought that would literally be paying to win against players who were using less powerful ammo. Mechwarrior Online is borderline p2w with the Hero Mechs that offer unique loadouts, though PGI.
If WoW offered ways to make your character more powerful using real money that couldn't be earned by playing the game normally (i.e. paying to win) then it would be pay-to-win, if not then it isn't.