So your warrior wont turn in to a tanking machine instead of the semi-dps that you do whilst levelling?
Your rogue will actually use stealth to sneak around whilst raiding? k .......
Your shaman wont sit there casting ... heal .... heal .... drop totem ... heal ?
You're going to use your pet for something other than sacrificing in raiding? (bar 1 warlock for imp
sometimes)
Your druid will heal .... heal some more and then heal a bit more and if youre very very lucky run around as a bear and sit there tanking. Slightly different to the cat form dps with a bit of balance in there whilst levelling.
Priests dont get shadowform by level 20 so the usual levelling wont reflect endgame, as thats either shadowform or pure healing.
Hunters pets are used endgame yes, hunters at not often used in raids though so you might aswell only go hunter if you dont want to do any pve.
Paladins like to beat on mobs with a big 2 hander and awesome looking spells whilst levelling ..... sorry but you'll be sat there blessing an entire raid every 15mins and spamming flash of light.
Of course none of this is a relevant change if you compare sub level 20 to end game playstyle. i must be mistaken