- 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.
It's important to note that many people use "amazing" as slang to mean "very good," "awesome," "excellent" etc. But other forms of the verb "amaze" are taken literally.
So your two sentences are both grammatical, but have different meanings because of the colloquial meaning of "amazing"
I was feeling amazing at the party ...
means that you were feeling exceptionally good while at the party. The party likely put you in high spirits.
I was feeling amazed at the party...
means that you literally were amazed while at the party. Perhaps the party was a lot better than you expected, and you were impressed by the quality. Or maybe you were amazed by a illusionist/magic show.
Best Answer
Neither is something that I can imagine a native English speaker saying. But both are grammatical, with different meanings.
I suspect that what you meant to say is "Keep amazing me!" - an instruction to your readers that they (the subject) should carry on amazing you (the object).
"Keep me amazed" would have a similar meaning: "Keep me in the state of being amazed (by sending me amazing things)".
"Keep me amazing" would mean "Keep me in the state of being amazing". Your friends might think that you are amazing, but this is nothing to do with things amazing you - it is a description of you.