I'll make a simpler list for you.
Allowed: Sound and texture mods.
Disallowed: Everything else.
Anything that changes the model, collision, transparent-ness, explosiveness etc. of objects is not allowed.
Anything that changed handling of players/vehicles including custom animations, is not allowed.
Anything that changes how weapons operate, is not allowed (you can only change how they look).
The most basic thing to note is that you can't use mods that either a) can affect other players (such as you turn a Rancher into a Kart, they will still see a Rancher but it'll be sunk into the road) or b) gives you an advantage, such as making walls invisible or making nametags visible from any distance.
So I reiterate, stick to texture/sound mods.