Well I've played just about every ORPG there is.
I think in my opinion from most important to least (remember just my opinion don't hate D
-Balance (so god damned crucial); this includes leveling, attributes, spells, gold, enemies, quests, etc.
-Lots of quests, including your standard slay this and that, or kill this guy, or retrieve my item (HOWEVER); inside quests you can put some that are puzzles to solve, or some that are triggered by riddle solving and typing it at a certain location (cool stuff like that keeps me very entertained!)
-Nice terrain that is varied yet not to overfilled you can barely see anything. Custom terrain is so nice to look at (gaia and TKOK for example)
-Hero variation, I find it nice in Gaia that there is only a total of 10 classes, that is enough, maybe 4 base classes x3 specilizations are better so there's 12. Please do not do 4 base classes with 1 options to advance at 4 different advancements like twilight.
-Boss fights that aren't basic tank and spank, have them do cool things like summon minions every 30secs at 75% hp, gain a buff at 50% and then turn invulnerable at 25% hp and charge around the room for 10seconds 1 hitting anyone that doesn't avoid it or something. Stuff like that is awesome and keeps people on their toes and doesn't let high levels carrying people just afk at the keyboard.
-Don't go with higher numbers, i.e. heroes getting to lvl 200 with 2k agil and shet, it's stupid and harder to balance in my opinion. I think a level cap of 50-60 is ideal.
There are more but I think those are most important
GL if you decide to go through with it