Here's some more:
Wandless Magic: in rare instances of the book, magic is done without the benefit of a wand (when Harry makes the glass disappear in the snake terrarium, or when Professor Quirrell surrounds the room with fire). And according to J.K. Rowling herself, "You can do unfocused and uncontrolled magic without a wand (for instance when Harry blows up Aunt Marge) but to do really good spells, yes, you need a wand."
D20 Interpretation: There are many ways to handle this, but I think the easiest would be to use a feat:
Wandless Spellcaster (General)
You can cast spells without a wand.
Benefit: you can cast arcane magic without a wand. Such a spell uses up a spell slot one level higher than normal, and requires a full-round action.
Animagi: Some wizards in the Potter universe have the ability to change shape into a particular animal. Rita Skeeter can turn into a beetle, and Professor McGonagall can turn into a cat, for example. This ability seems to be a choice rather than an inborn ability: when their friend Lupin contracted lycanthropy, Peter Pettigrew, James Potter, and Sirius Black decided to become animagi so that they could take animal forms as well. Peter chose the form of a rat, James chose a stag, and Sirius chose a dog.
D20 Interpretation: This would probably be best handled with a feat.
Animagi (General)
You excel in the arts of Transfiguration, and have learned how to change your form into that of an animal.
Prerequisites: wizard level 5
Benefit: choose one animal of Small or Medium size. You gain the ability to wild shape into the form of this animal, once per day, as a 5th level druid.
To answer the "why can some wizards change into smaller animals?" question, I suggest the following:
Improved Animagi (General)
You are among the most skilled animagi in the wizarding world.
Prerequisites: Animagi, wizard level 11
Benefit: choose one animal of Tiny or Large size. You gain the ability to wild shape into the form of this animal, once per day, as an 11th level druid. This ability replaces (does not overlap) the Animagi feat.
Lycanthropy: Professor Lupin is afflicted with an incurable magical illness that forces him to become a werewolf. Even with centuries of magical research, the wizarding world has failed to find a cure for this painful and debilitating illness, and those who are afflicted with it become social outcasts.
D20 Interpretation: unless the illness is diagnosed and treated before the first full moon after exposure, lycanthropy is permanent. Spells and magical effects that cure magical diseases are only able to suppress the illness for one hour per caster level. Mortal magic cannot cure lycanthropy.
Raising the Dead: Dumbledore insists that no magic can raise the dead, and has mentioned this on more than one occasion. We don't know if this is because it is simply impossible (as with modern science), or if it is impossible because of what would happen to the persons casting and receiving the spells...but for whatever reason, it cannot be done.
D20 Interpretation: I think the entire family of raise dead spells (raise dead, resurrection, true resurrection, reincarnate, etc.) should be scrapped.