I notice a lot of naysayers of Old school have a something in common.
They focus on the extreme bad...... I agree !
Extreme ideas such as 30 man 12 hour raids, and camping 3 days for loot, or forcing the player to play 6 hours... Their not for everyone, infact only for few.
Anything extreme should be optional and a very small percentage of the game.
To implement anything extreme the game has to be "large enough" to support all types of play short, long and extreme. The player should be able to play the entire game of any of the three categories all the way to it's end separately.
An mmorpgs must be large...... This is the key to EVERYTHING !..... It's why newer mmorpgs fail.
Vanilla World of Warcraft was that large game, it's why it's always number one. It supports everything, short long and extreme.
Everyone can co-exist. Time permitting the short player can play long or extreme.... It's called do what you want.
In order to test a healthy mmorpg:
The players should be able to play hours without having to do any combat..... Enjoyable content.
Unfortunately everything newer would fail this test.