- It is not waterproof. That is the only thing that is bad about it.
This is the most natural sounding phrase. There is only one thing that is bad about it. You state the bad thing, and reiterate that it stands alone as being a fault.
- It is not waterproof. That is everything that is bad about it.
Everything means all of the things of a group - it refers to the collection, and the collection normally has more than one item within it. While it isn't horribly wrong, and would be understood, I would not use this.
- Every thing vs. everything
Every thing highlights each of the things as it's own entity, rather than as a collective. Imagine if I said Make sure every thing is put away. I am acknowledging that different things have different repositories, and asking for the listener to exercise care that each item is treated with care. 999 of the 1000 items being stowed, is NOT every thing.
Contrast with Make sure everything is put away. Now I am not focusing on these objects as separate things. As a collection, I want them put where they go. And if I put away 48 of the 50 things that were out, I might be OK saying I put away everything.
I can picture a circumstance where I would use every thing in your phrase.
A: Tell me everything that is wrong with this backpack.
B: It isn't waterproof.
A: I asked you to tell me everything that is wrong with it.
B: "That is every thing."
That is everything I have to say about this.
Best Answer
Here you don't use an article because you are using behaviour in its uncountable sense – referring to his behaviour in general.
You would use an article if you are referring to a specific behaviour: