"Now, we're off to the next version. I can't tell you what your #2 and 3 suggestions were, but I do know that they're on the way."
Cross-platform
While Google Talk is available only for Windows, Google will release versions for Mac and Linux. Microsoft doesn't intend to release Windows Live Messnger for Mac (there's just a limited Microsoft Messenger for Mac), while Yahoo has a very old Yahoo Messenger for Linux.
Open protocol
Google Talk uses an open protocol, XMPP, for instant messaging and Jingle, an extension of XMPP, for VoIP. While Yahoo Messenger and Windows Live Messenger are interoperable, the real basis of IM interoperability was put by Google Talk. There are also plans for AIM interoperability. In fact, you can already use GTalk to chat with your friends from other IM networks, but it's pretty complicated.
Simple interface
Unlike other IM clients, Google Talk has a very simple and clean interface. There are no ads, no menus with too many options.
Chat in a browser
While there are services like Meebo, that allow you to use multiple IM networks from your browser, Google has implemented this for Google Talk in Gmail, treating IM conversations the same way as emails. Google also records the chat logs and makes them searchable in Gmail.
Encryption
Google says: "Google Talk currently does not encrypt chats or calls. But we are working hard to make many improvements to Google Talk while it is in beta, and we plan to fully support encryption of chats and calls before our official release." Yahoo Messenger and Windows Live Messenger don't offer encryption.
Mobility
Google Talk is included in Nokia 770 tablet, and can be used on a BlackBerry (Yahoo Messenger is also available).
File size
You can argue that it has less features, but Google Talk setup has 1.45 MB, Yahoo Messenger has 9.5 MB and Windows Live Messenger has 15.3 MB. This is a good indicator for bloatedness.
I'll conclude with a small description from a review at FileForum:
"Lightweight and simple. I think it's great."