I think Halo's big contribution was popularizing the regenerative health concept, and making online FPS playing a big deal (for consoles). I love me some Quake 3 on Dreamcast, but it was more ahead of it's time than game changing.
Golden Eye and Turok I think were the big pushers for FPS on console though.
I see Halo as the Final Fantasy VII of the FPS genre. No, it wasn't the first, it wasn't the best but it did some things really well and got the gaming populous at large to take note, and make the genre popular for years.
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