This is one of the biggest thing i look into before i choose to invest my time in an MMO.

Boss battles should be EPIC enough so i can take it seriously ,

to further explain my thought, i loved MMOs with world bosses that spawn once everyday, some MMOs i played also combined things beautifully , There were 48 Hour Spawn boss , 24 hour ones , 12 hour ones , 4 hour ones and lastly even 30 mins spawn Mini bosses.

Having said that , world bosses make it incredibly difficult for casual players or people who just cannot catch a break in being online during a Boss spawn to experience those content and the progression it brings.

From the short experience i had with BlackDesert (Region Locked for me ) , i would say that i loved the idea that individuals had the ability to use Summon Scrolls to initiate a boss encounter.

It would be nice if an MMO had a mix of World Bosses , Guild Bosses and Individual bosses.
The benefits they give can vary accordingly to the rarity of encounter and difficulty involved.

Items/Gears :

A lot of new MMOs have been trying really hard to re-invent Itemization and gearing up , i never understood why they are doing this.(Personal view)

I have played a lot of AAA MMOs and a lot of Indie-ish MMOs , and shit tons of Korean ones ,
Majority of them had no problems with Gears and Items , the traditional approach always proved to be one one of the main attractions of those MMOs , a lot of MMOs literally run on the P2W based off their Items/Gears.

Having said all that , my hope for any MMO i play is that things are kept fairly simple.

The Grades of weapons (level limits) , Their rarity , Blessing/Enchanting weapons ( +1 ~ +15 for example ) and finally Engarving runes /Inscriptions of special abilities .

MMOs that pair Items with Bosses and mini-bosses and various significant AI-Mobs ingame are really nice.
Thunder Dragon boss dropping Weapons / Crafting materials towards weapons and armors towards Similar attributed weapons/armors and even possesing the boss' name e.t.c are very tasteful . It makes the MMO world Less Complex and memorable

Since am talking about items i would love to point out the importance of an items/gears for a game , example : Diablo Franchise!! , Diablo Online would be a smash hit when it eventually comes out.

Factions. :

I love factions in an MMO for the very obvious reasons , but i hate when its overdone . ( 4+ Factions).
I love competing in Open Word and War area scenarios , competition for "Rule" over an area of importance towards progression along with Bosses makes things really exciting .

These are my views on some of the core things i expect from and MMO , my views might be flawed and my expectations might be over the top, i look forward to reading the views others hold about these things in an MMO .

Edit: Have not proof-read the post , please look past bad sentence constructions / grammatical errors.