What make it fun?? when the mechanics no affect in a negative way the gameplay (i hate d&d mechanics on crpg).
When you found interest npc on the crpg, that you can even admire (dakkon on ps:t or boo the giant hamster from the space).
When main quest make you go forward, like reading a awesome book, that in all moments it make you go forward even is too late and you have to school next day, you know like reading the hexer or X novel you like. (irenicus make me go forward on Baldurs gate 2, specially after the asilum scene where yashiko, dont remember his name, die).
When side quest affect the main quest in some way, i mean dont make a distintion that all crpg do with the modern rpgs, i mean everything on games is always MAIN and SIDE, there is no relation betwen then, even in a little scale and it sux
When you do something on game and it affect in some way the world, and you can see the consecuences of your decisions.
When in my case, got mature stuff, i not mean porn, i mean real issues of life like choose betwhen help someone that really show humility and ask you for help him to get X, not the usually i give you money and go to the dungeon X kill the rat F and come to me for the money, if you do a check stat then you can get more money...
Lot more stuff i no remember.
And for god sake, the stupid thing about save the world is the worst thing you can put on a rpg, is nice if the quest mean to just save yourself or a friend from childhood, or a familiar. BUT THE WORLD NO, it dont deserve my time.