I came back from roughly 2017, the user interface has become leaps better and it's the change I love the most:
>The items now stack in the inventory, each stack only counts as 1 inventory slot;
>You can buy/sell il bulk in the auction;
>You can move entire stacks (or # amount) to/from stash;
>The notifications that appear on screen (jewel farming*, kill 200 mobs quest) no longer lag behind and don't endlessly clutter the screen long after you're done;
>The crafting interface has been improved to minimize scrolling, since a lot of craft able items have been added.

*(Jewels replace gems and go from +1 to +9 stat; there are also health, mana, armor, damage and crit jewels.)

Other gameplay changes that might excite you:
>There are more minor events;
>There are now skill masteries that improve them (not just buffing damage), and ultimates that cost 10 skill points each;
>There are hotbar slots on the screen for super quick loadout and pet swapping (or chest/elixir opening) without accessing the inventory;
>There are gear sets: if you wear all or a # amount of pieces you gain a buff (some are restricted to one expansion);
>There are Haste sets that give you 200% movement speed, they're really useful and I can't live without it now that I've had a taste;
>Your gear can be "awakened" to give you additional buffs (+% stats, armor, movement speed, boss damage, etc.). This is really important if you're an avid pvp player or leaderboard runner, otherwise it's great for pve;
>There's a yearly, energy-free event where you can farm almost any egg in the game, from common to arcane;
>Leveling for the first ~50 levels is quite easy, since story quests give insane amounts of experience.

Things that might turn you away;
>Leveling gets really tedious at higher levels (100k points per level for the latest cap). However, several daily quests now give exp ranging from 150 to 1,750 depending on the expansion; they can give you 30+k exp in one day. You can get free 50% elixirs by watching a 30 second ad. A player told me they stack with 60% and elite runner elixirs for 60k exp/day, but I haven't tried it.
>Energy essences and crafted energy kits are deleted at the end of each event, no more hoarding them.
>Awakenings made low level twinks really hard to kill, because of ridiculously high armor.