You will want to think ahead, let's say you hold a Closed Beta, and loads of ppl see it. What will grab their attention to give it a short ?
An MORPG. Alright, what next. There will be combat. Good. What kind of combat ? Stupid mindless mobs running around, waiting to be killed ? Are there dungeons ? What 's in the dungeons ? More monsters ? Are they just all over the place ? Are they guarding rooms or treasure chests ? Are they stronger than mobs outside of dungeons ? Or are they just coming at ya in larger number. Are there bosses ? How to actually beat these bosses ? How hard are they ? Can a player solo them ? Can 2 people beat it ? Or do you need 4 ppl + to beat it ? Will there be incentive for players to group up ? Will there be storyline and quests ? What kind of story is it ? Is it the kind that most players will just skip the whole thing and read quest objectives instead ? Will there be exploration ? What do players gain from doing it ? What else are there for 'player interactions'. They group up. Ok, what else. Do they trade ? What kind of stuff do they trade ? What's the point of it ? Do you want some form of economy in the game ? Ok. What about crafting and resource gathering ? How many professions are there in total ? How many professions can a player obtain ? Will players chop down woods, saw it, put together a Hunting Spear ? No ? Think up more details.
After all those questions and more, you will want to ask yourself; now, what's endgame content ? What will keep players logging in days after days after days ? What will keep them from quitting ? What do they gain or what do they do at the end ?