How to best sumarize...
Upside : New races/Raids/Islands to explore, whole new questline that puts the Alliance and Horde on a direct warpath again (Teldrassil gets burned down by Sylvanas, the new Horde warchief), a lot more lore and addition of new scenarios called "Island Expeditions" for 3 players PvE or PvP that reward tons of pets/mounts and the new Legendary currency Azurite, new 20 player PvE battlegrounds, proper DX12 implementation with some nice FPS gains. Other minor stuffs.
Meh side : 1-120 levelling time increased (almost back to old days), Tanks and Healers redesigned to take more damage so as to force players to pay attention and actually use cooldowns (no more easily grabbing whole rooms while the healer dps/afk), Tank threat and damage down which is hilarious when doing Mythic+ with Skittish attribute, Druids are still meh overall even though they got buffed 2x, Demon Hunters are still kings, heading in for the 3rd raid and still no flying in the new areas, a lot of quest-lines/profession stuff gated behind reputation (this might actually the most annoying part for the casual player because it will take you on average a couple months to get Exalted with all factions).