Well, for one, mobs now have varying levels. So, you might decide to not pull the level 29 kobold, and pull the level 27 instead. Those levels make a big difference.
Classes have a lot less in health sustain, so this requires the use of more skills to get kills faster.
Resists are more plentiful, crits are more sparse.
As a warrior you are a few parries and misses away from a normal pull turning into a game of survival.
Retails decision to even the level of mobs to you is baffling to me.
Playing through BfA my character was a buzz saw, chain pulling mobs, self healing, using the same 3-4 skills each pull.
In retail, sometimes you have to hamstring kite and bandage, or pop an intimidating shout and run and pray. The difficulty increases the amount you have to rely on being tactical.