I think the most amazing of all the upgrades from the NES to the SNES was the vast improvement of sound and the wide variety of soundchips. Sometimes the quality of a particular soundtrack depended on whether or not the soundchip sounded well, and if it had varied enough sounds to make it not sound repetitive or weird or whatnot.
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Nominate Left Pong Paddle vs. Right Pong Paddle for the Rivalry Contest!