(thanks to Cyberglass and Kingler5 for planting this idea in my head)
Steel has always been a very good defensive type, but when Steel-type was made in Gen.II, we did not yet have the physical/special split that was introduced in Gen.IV. Now that we have the split, it is time for Steel, a Defensive type, to have a Special Defense counterpart type. This is where Glass type comes in. It could alternatively be named Crystal Type, but I haven't decided yet.
========= Type Effectiveness: =========
-=Super Effective (x2) Against=-
Electric
Flying
Poison
<> Reasoning: Glass is officially considered an insulator, and glass may be the number one killer of birds in urban areas other than pollution. As for poison, glass is the substance usually used to contain it. This also serves the purpose of giving two highly defensable types, Electric (1 weakness - ground) and Poison (2 weaknesses - ground and psychic), another succeptability.
-=Not Very Affective (x1/2) Against=-
Ice
Rock
Steel
<> Reasoning: A fragile substance like Glass is not going to hurt harder substances like Ice, Rock, and Steel very much. Ice could also use another resistance, since it currently only has one (itself).
-=No Effect (x0) against=-
none
<> Reasoning: All types have some succeptability to glass
-=Weak to (x2)=-
Rock
Steel
Dark
<> Reasoning: Rock and Steel will almost always shatter glass. Dark is supereffective simply because Dark is only effective against two types, and it is usually the sneaky thief that will break a window.
-=Resistant to (x1/2)=-
Grass
Electric
Fighting
Poison
Ground
Psychic
Bug
Ghost
<> Reasoning: Glass is supposed to resist a lot of previous "special" types, and as we already noted, it is an insulator and can be used to safely contain poison. That makes Grass, Electric, Poison, Psychic, and Ghost explainable. Ground is easy to explain, since even a sandstorm does little more than put minor scratches on glass. Bug and Fighting resistances are less obvious. I wanted to keep with the tradition of Bug being really good when it's good and not-so-useful in other situations. (And honestly, there are a lot of dead bugs on my windshield when I drive in the summer.) Fighting... well, Fighting is already super-effective against 5 types. And really... who would be dumb enough to punch through glass? It does just as much damage to the puncher as it does to the glass.
-=Immune to (x0)=-
Water
Flying
<>Reasoning: I traded off Steel's 11 resistances and 1 immunity for 8 resistances and 2 immunities... a defensive combination not seen before. As for water... you can immerse glass in water for a hundred years and not see any deterioration. Glass is also used to shield against wind, and, as I stated above in a rather tongue-in-cheek fashion, lots of birds die by flying straight into glass. Usually, the glass isn't even scratched. I considered making Glass immune to Poison, since glass vials are really the only safe way to contain poisons and acids for any length of time, but poison is already underpowered enough as it is, so I simply made Glass resistant.
-=Normal Effectiveness (x1)=-
Normal
Fire
Ice
Dragon
<>Reasoning: First, I wanted more types to be normally-effective against glass than there are against Steel since I am giving it two immunities. Because of that, Normal and Dragon have normal effectiveness, keeping with tradition of those two types. I considered making Normal super-effective, just to give normal a super-effectiveness, but decided against it. Fire and Ice were tricky. I considered making glass weak to fire, since fire can obviously melt glass, but fire is already the most powerful of the starting three, and as one person whom I can't remember pointed out, if these games were really true to life, Fire and Rock would beat everything. I guess I can say that Fire is used to shape glass, but it can't actually destroy it. As for Ice... it can be used to make glass brittle by hardening it to the point where it is almost solid (glass is considered a liquid), but glass is also used as an insulator against the cold. So I settled for a middle ground. Also, Ice doesn't need another super-effectiveness.
-=-=Overall Type Chart Reasoning=-=-
I tried to preserve the integrity of certain "unusual" types like Normal, Ice, Bug, and Dragon, while making Glass interact realistically with the other types. The current type chart, combined with the fact that glass will tend to have high special defense and low defense, makes Glass a strong defending type without overpowering it. I had to trade off a few "obvious" explanations and add in a few non-obvious explanations in order to balance it, but such is the nature of the beast.