I highly doubt Notch wold notice this, let alone add it the the game. But its a very good idea.
Here are my suggestions on microphone chat:
1. Add a microphone chat system
2. Press a button to toggle client microphone chat (z as default button) if you have a microphone!
3. A new sound slider for microphone volume (hearing others)
4. If minecraft can detect sound comming through a microphone, the players' nametag would become red
5. New server.properties commands:
a. Mic-chat = true/false (if players can use voice chat on the server)
b. Mic-distance= 1 to 256 blocks (leave blank for unlimited range)
- Mic distance measures how far away you are from another player's reach. Example:
If Mic Distance is at 16, you can't hear a player's microphone if he/she is more than 16 blocks away from you. Making voice chat more realistic. At 16 block distance, the mic volume fades away.
I love the idea of a social aspect to Minecraft, but since one does not currently exist, and it doesn't look like it will for some time (if ever) I've searched the nets and found sites that fill this void.
For instance there's the social networks minecraftr and also www.minecraftrevolution.com (which is a little bit newer and better laid out in my opinion).
Both have most, and in some cases more, of the features you stated above, it's just that they are not directly linked to Minecraft and are 3rd party sites. I'd definately recommend checking em' out though, it adds that missing social-side to Minecraft.
Yes, when Minecraft is released on steam a lot of this won't matter anymore, but the ability to trade and etc would be useful. (Though if I read it correctly, OP is suggest trading between worlds? Which would be a bad idea.)