It’s a significant update. The Arduino UNO and Arduino Mega 2560 update the Duemilanove and Mega respectively. They now use an ATMega8U2 instead of an FTDI USB to serial converter chip.

[Andy] stuffed some more RAM onto an Arduino Mega and his three-part walk through on the design, construction, and software is a great read and one of the more ‘hard core’ Arduino builds we ...

and Mega 2560 don't support the Debugger feature as they're powered by an AVR microcontroller. How Are The Boards Used With ...