WoW Forever changes quite a few core mechanics that druid players from Classic have grown used to. I will go over some of the most impactful ones
Your weapon matters
When gearing up, you will get your biggest upgrades from new weapons. What you should pay attention to is the dps value on new weapons. If it is higher than your previous weapon, then it will most likely be an upgrade.
Note that the weapon speed does not matter for Feral druids. Your swing timer is based on your form, not your weapon, and effectively counts as a 2handed weapon.
For bear, your swing will be 2.5 seconds
For cat, your swing will be 1 second
Additionally, on-hit effects do proc and weightstones do work for Feral druids.
Quality of life
In WoW Forever most in-combat consumables and abilities are now usable in forms.
The things usable in Feral forms include (but are not limited to):
- Combat potions (Health pot, mana pot, armor pot, dps pots...)
- Healthstones
- Food/Drink
- Bandages
- Goblin Sappers and EZ- / SAF-T Items
- Racials (including War Stomp from Tauren or Shadowmeld from Night Elfs)
- Faerie Fire (which will trigger a 6 second cooldown when used in bear or cat form)
- Nature's Grasp (which is now baseline and also usable indoors)
- Interacting with npcs and objects in general, including things like collecting items, accepting quests or takking flight points
Rage, energy and mana
Rage
Rage is normalized, meaning that for every hit, you will gain 11.25 rage and for every crit, you will get double the rage from a normal hit meaning 22.5 rage.
Rage from damage taken follows a new formula. It should result in a bit more than 2x the rage it used to generate in Vanilla.
Energy
Energy no longer goes up in ticks of 20 energy at a time but now gradually fills up at a rate of 10 energy per second.
When speccing into the 
Furor talent (row 1 in the Restoration tree), you will see some additional changes to how energy behaves. At 5 talent points, you will keep the energy you last had in cat form when shifting out and back in, plus you will keep your energy regeneration while not in cat form. The exception is that the energy won't keep regenerating when spending time in bear form.
Mana
Mana will begin to regenerate after 5 seconds of not using a spell. Shapeshifts do count as spells in this context, but bear or cat abilities do not. This means that you will regenerate mana while staying in form. It is advised to monitor your mana while you are shapeshifted. On the default UI, you can enable your mana bar by enabling the Personal Resource Display under Options -> Gameplay -> Combat.
Damage over time (dot) and healing over time (hot) effects can crit
In WoW Forever, dots and hots are able to crit as a baseline effect. This is great news for all druid specs.
Changes to powershifting
Powershifting is no longer a fixed part of the Feral rotation. It is still a useful tool to get out of rooting or slowing effects, but is no longer used to gain damage.
As a replacement, there is a new talent in the Feral tree called 
Shifting Power which acts similarly to how powershifting used to work. It costs mana equal to the mana cost of your shapeshifts and will give you 40 energy. It's baseline cooldown is 16 seconds which can be reduced down to 8 seconds by spending 2 more talent points.
"Feral Combat" as its own weaponskill
This is a new system that you might passively notice but won't actually affect your gameplay in any significant way. The same way other classes level up their weapon skills, 
Feral Combat 
Druids now also level up their own weapon skill called "Feral Combat". This skill is the same for bear and cat and is not affected by the actual weapon you use.
Polearms
At level 20, druids can learn to use polearms for the cost of 1 gold. This wasn't the case in Vanilla and opens up some very nice new weapon options.
Omen of Clarity
Omen of Clarity is no longer a buff and is instead a passive that can be learned by any druid spec at level 20.
Class-Quests
Skyborn Druids will receive the quest for 
Bear Form from the Druid trainer on Zephras Isles. This is not the same quest as Tauren and Night Elfs will have to complete.
The Aquatic Form quest that is available from level 16 now also provides an early idol in 
Idol of Shifting Tides.
Once you hit level 20 you will have to complete a quest to unlock cat form. The quest starts at the druid trainer in Darnassus for Alliance and at the druid trainer in Thunder Bluff for Horde.
Druids get a real resurrection spell
Druids used to not have a proper way to resurrect people and were stuck with only 
Rebirth, which triggered a cooldown. Starting at level 12, druids can now learn 
Revive.
Druid
Feral
Cat Form for higher damage, while still having access to a variety of healing, damaging and utility spells while in humanoid form.
Paladin and 
Warrior. Historically, 
