* colors for labels
* detaching messages from a conversation
* Jabber transports (these could be used to chat with people from other IM networks, like Yahoo, MSN, AIM). You could already use these transports to connect Google Talk with other IM networks, but you have to use a third-party server and another IM client to configure the transports. Being able to chat with people from other networks, which are much more popular than Google Talk, will make Google's instant messenger more useful.
An extract from Gmail's JavaScript code:
It's also interesting that the new contact manager from Gmail lets you enter usernames for many IM networks:
What else would you like to see in Gmail 2.0?