Trillian currently supports everything that the AIM client does. This includes:

Buddy Icons (And browser integration for new ones)
File Transfers
Direct Connect
Multiple user chat
Warnings
Away and back messages
Idle time
Font, color and size changes to the text
Emoticons

It also has the added benefits:
No ads
Better support, post a problem or suggestion to their fourms and there is a good chance someone will pay attention to it
Secure messages with other Trillian users over the AIM network
ICQ like message sending if so desired (where the window closes on send, tray icon blinks instead of a message popping up)
Containers, to have one taskbar entry represent whatever Trillian chat windows.
Tons of skins, and skin support built in

I switched to Trillian from ICQ just because I wanted to have a better client. And now that I have, I have picked up several new people on my contact list from the different networks. It's well worth the switch even if you do only use AIM.