Hardly any content was nerfed during vanilla. It was mostly bug fixes and fixing broken mechanics (such as C’Thun) or the conversion of the 10-man dungeons to 5-mans. Content “feels” nerfed because of: Updated talents and abilities, updated/better itemization, 16 debuff slots, and15 years of knowledge about the game as well as experience playing a significantly mechanically harder WoW.
I am also not counting the 2.0 pre-patch as being “part of vanilla.”