I had a pretty random idea that I'm pretty sure that no other MMORPG has done (correct me if I'm wrong).
If you stand in the middle of a city center on Earth and you listen out. You can hear the constant chattering all around you. You can hear the noise of cars, footsteps, people shouting over all this. As you walk around and travel to a more quiet, secluded spot, you can hear the distant sounds, or maybe even nothing but your own footsteps and shuffling.
You stand near Harnquist's Smithy...what do you hear...? Thats right, nothing but the 'inthedarktown.ogg' music track looping in the background.
Now the chat window shows that your character is not deaf, he can hear people speaking from 10 meters away. He can even hear that person who's shouting the closest n00b all the way over by the tavern. Yet, you hear silence. For added realism, I think the client should play certain sounds depending on where you are, and how many people are in the vicinity.
In a way, a movement in this direction has been made with the sounds of birds in the wilderness and you can hear the rain and thunder when the GMs are feeling happy. But I think a chattering noise should be heard when in populated places. Say, when there are more than 20 people in one spot, the client plays looping sound clip of lots of people chattering. When there are less than 20, but more than 10, there should be a clip of a few people chatting. Less than this, you should hear a couple of people chatting.
Obviously, this should also be relative to the area people are in. Perhaps in the Arena, it should echo. In the wilderness, you shouldn't hear much but a whisper. In Hydlaa, it should be loud, city type shouting.
In addition to the chatting noise, if you are near the smithy, should you not hear the flames of the furnace? Should you not hear the clanking and smashing of hammers and the hiss of the red hot metal when they are shoved into the quench tank? When you are in a more secluded spot, your character and characters around you should emit a shuffle of cloaks, a clank of armour, a thud of heavy footsteps.
These voices I believe should all be in a strange language that only people like the Sims or Jabba the Hut could understand. There would be nothing worse than sitting there listening to the same conversation over and over and over again.
And of course, for those select few who find the noises of chatter to be 'cramping their style', there should be the obligatory tick box to turn off background noises and back to the good old guitar and flute.
Right...'nuf elaboration. Use your imagination.
PS: I know it will greatly increase the download size with all those extra sound files! So don't go on about it!