3D graphics really enhance the game,  so it kinda is a 'must-have'. There are games that don't use 3D graphics which are somewhat popular. Adventure Quest is a 2D side scroller, and is popular because it is so easy to start playing, and is fairly easy anyways. If you don't want to do 3D graphics, 2.5D is fairly nice as well. I have played a 2.5D game called Crystal Saga, which incorporates a flat back round with 2.5D characters/sprites, which does not look bad at all.

The story is very important!! A game that doesn't keep the gamer hooked in is not going to be successful. I myself have been trying out many MMORPG's, and not many have hooked me in. I recently saw that Wizard101 and Pirate101 where very successful MMORPG's, and I tried them out. Boy...I was at a loss for words. It looks like a kiddie game, it really does. But when you play it, you are so immersed into it...the story line is pretty predictable, but the events that take place really keep you going. The story is like a book you absolutely cannot put down. The graphics on both were in my opinion, flawless. The controls were fairly easy, seeing how you don't really fight until interaction, so there's no need for hot keys and such. 
A good story would be something that is always entertaining and not too hard. You don't want to come up to a boss you can never beat and is always boring. What a drab. You can also smother the game with seemingly endless side-quests, such as what RuneScape has done. It may be a little boring at times, but usually, each side-quest offers its own style...some with humor, some with action, some with even romance. Keep that dynamic inconsistency in the game, and players will be hooked, trying to guess what's next.  

Difficulty is big factor as well. This part is pretty self-explanatory, and really common sense. You don't want to have a game too hard, but not too easy either. Present some pretty complex challenges here and there, and balance out with some easy quests and such. Quitting because it was legitimately too hard doesn't bode too well.  

Diversity.

- Character customization
- Classes
- Quests (not always the grinding quest, or the collecting/harvesting)
- Items (weapons, armor, costumes, etc.) 
- Maps (if there are multiple maps, make them very different from each other, like in Wizard/Pirate101. Adventure Quest does not follow this well however.) 
- Events (special events that are on at certain times, dates, etc, should never be a drab...make em inconsistent, keep the player guessing!) 

Anyway, those are the few that I can think of. It seems that the stuff I posted was advice, but those are just things I look for in a game. If you take it as advice, then great.

Good luck with your MMORPG making!