I will add more when I think of them, but so far:

-- Shield star level does not effect what tier you are put in.

For example if you have full 3* equipment plus a 4* shield, you still get put in Tier 2.

-- Infinite sleep vials.

If there were infinitely re-spawning Tier 1 sleep vials in some maps, it would make Jelly armour's sleep protection not entirely useless, and if they took a while to re-spawn, then they would be no more overpowered than snowballs.

-- Health dropped by defeating an opponent increases with tier.

For example, if you defeated someone in Tier 3, they would drop one of those hearts with a smaller white heart in the middle which restore 3 health bars.