Applications don’t bounce on the dock anymore, they just launch instantaneously (even Photoshop).
Quicksilver starts in under 2 seconds and when I press the magic-key combo to call upon it, it immediately awaits my command. Oh and it finds and does whatever I want it to do before I have time to blink (i.e. Quicksilver just became useful).
Neverwinter Nights runs beautifully with every graphical bell and whistle turned on and with a pornographically high resolution. It is fast and gorgeous.
Homeworld 2 is just indescribable. Pure beauty.
Yes, the new iMac changed my life. ;-)