Compare these two sentences...
"It wasn't too bad!"
"It was quite good!"
Do they mean the same thing?
I would say yes, more or less.
Do they come across in a similar way?
I would say no without question.
One is clearly negative, whereas the other is clearly positive.
I once had a conversation with a friend on the use of negatives in general conversation and the outcome was interesting in the simplest sense.
We agreed that through using negatives, what's reflected in speech is that the mind filters information differently - negatively as opposed to positively, which in turn gives a negative outward impression.
Why say one thing negatively when the same can be said positively?
So, next time someone asks "how are you today?", why not reply with "I'm fine!" as opposed to "not too bad!"