I sometimes hear some people saying,
- it's not that good "of an idea" or
- it's not that big "of a deal".
Is there a difference between above statements and statements below?
- It's not a good idea.
- It's not a big deal.
If an idea is not good then it is not good. But an idea can be good but not that good of an idea. That here restricts how good something is, meaning it's not as good as someone claims it is, either now or previously.
I thought baking the chicken in coca cola would be a good idea. And it was a good idea, but just not that good of an idea (as originally thought).
So it was still a good idea but not as good as originally thought.
Sometimes we might say this as a step toward deciding or confessing that it was indeed not a good idea at all. Or someone who doesn't think that it is a good idea might say it's not that good of an idea instead of just saying straight out that it's not a good idea. Anyway, sometimes we can't always be sure if our different opinion will be in line with the final verdict.
The concepts apply for deal and for a huge amount of nouns. Giving George a TV for Christmas was a good idea, but it wasn't that good of an idea; it was more like a mediocre idea.