Few. Some people might even say there are no differences.

But to me... it's the glitches. Every Type: Null I've seen so far has had some strange behaviours or physical abnormalities. It could be anything from strange compulsive behaviours to odd physical reactions to certain things. I have a friend who has one of the Nulls and hers gets completely distressed if it doesn't have a teacup on its head. It might actually be a very specific teacup even.

Most of the people I've talked to aren't really concerned by these things. They say it is fine because the Nulls don't seem hurt by it and regular pokemon can also have abnormal behaviours or physical defects but... I'm not sure I can agree with that.