Quoted from Aurich:
Well for instance the rising cross is just a little toy animation, it doesn't actually do anything. But the opening and closing snake jaw actually changes the way the game plays (for the better), so I'd list that over the cross personally if I was making a short list. Along with the spinner.
But really, the lights are my most favorite feature of the upgrades, both the color changing inserts and especially the GI.
Ah yes. The snake jaw and spinners. I kinda wish I had the snake jaw.