I know Killzone was the one that started it. Then people started looking into other games and found they weren't playing at the specs advertised or stated by Sony.
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For the most part it just looks like the 30fps rather than 60fps are only big difference(except for Battlefield 4, which runs at 900p 60fps on PS4 and 720p and 60fps on XBOne). I expect this to change later in the generation.