I'd see Light as a fairly balancing type.
Let's see how.
Super Effective:
Looking over possibilities as both a photon and a purity form, these types come to mind:
Dragon, Dark, Ghost, Water, Steel
Steel is highly reflective and will leave a great glare and will burn to the touch. Water is too, and focused photons can vaporize it.
Dragon, Dark and Ghost are the 'evil' types vanquished by warriors of light... with orbs... warm destiny orbs!
So that's a bunch of common types countered there.
But where would it fall flat?
Fire, Ice, Electric, Poison
Fire and Electric would mostly be empowered. Ice needs a new resistance and Ice looks nice and shiny in the right light. Poisonous clouds deny sunlight: ask planet Venus!
Immune Types as I could see it.
Grass, Psychic.
Psychics are 'enlightened', thus above the light that is to be. Grass just absorbs it because its yummy sunlight!
As a defensive type.... things get interesting.
Weakness wise its a bit hard to say the way things could be.
Ghost, Poison, Psychic, Dragon
Simple enough. Poison murks up light types and ghost or shadows just block the light away. Dragons can use their mysticness to siphon it and psychics enlighten themselves even further! Lots of underrep'd types attackwise.
But what doesn't hit hard?
Fire, Ice, Electric, Ground, Rock, Fight
The physical earth cannot beat the light which enriches soil and the bright fires and sparks cannot really harm it. Can you punch intangible light without being Chuck Norris? Go ahead, punch a star! Ice will start to melt...ish.
Types Light could be immune to:
Dark
Shadows may pass, but you cannot absense the light without gravity!
So let's see what it does to the type chart:
New SE hits for Poison and Ghost! SHWEET!
Ice/Electric and Rock/Ground/Fight resist? Double sweet!
New SE dragon hit and dragon slayer? Awesome?
A middle finger for Spiritomb, Seadra, and all 3 Sinnoh Legendary dragons? Can't complain!
Grass and psychic get an immunity? Uh, Okay.
The only thing missing is something to give electricity a second weakness.
This post is Ironic because its the 666th response!