Forever replaces realm choice with rulesets: Normal, PvP, Roleplay, Roleplay PvP and, later, Hardcore. On the two PvP rulesets, other players can attack you while you quest. That changes which classes feel good.
Strong picks
Rogue: chooses every fight with stealth. Sap no longer breaks stealth, and Distract now lowers enemies' stealth detection.
Hunter: the longest range in the game, plus a pet and in-combat traps. Very hard to catch in the open world.
Warlock: Fear, damage over time and a pet. Excellent at punishing anyone who picks a fight.
Druid: hard to pin down. Travel Form, heals and shapeshifting get you out of almost anything.
Good, with friends
Warriors, 
Paladins and 
Shamans are strong with a healer or a group. 
Priests and 
Mages burst hard but are fragile when jumped.
Battlegrounds
Forever adds a new 15-vs-15 battleground, Battle for the Darkspear Islands. Healers are the most valuable players in any battleground, so a healing second spec from dual spec at 40 pays off quickly.