Dunno.

I really only get into JRPGs and some hack and slash stuff, and an mmo on the side. Hack and slash is pretty easy to keep fun, and the day they screw the ones I play up, then I'll laugh at their stupidity. As for rpgs, just get a great musician and awesome storyline matched with believable fun characters and an entertaining world and your set. Keep the battle system not too complex and don't waste so much time on graphics that any of the more important elements get forgotten. With an online game I usually chose them with my real life friends, so the social side is always set up before hand, but to make them enjoyable I like them challenging and worth striving for something. A great guild system is needed. PVP optional but never forced and classes that actually stand out far and wide from the others and each having a very high needed desire for raids.