A "big deal" can also be exactly what it sounds like: something meaningful or important. Usually in this context, "big deal" is used as a noun in a sentence as oppsed to being a complete (sarcastic) statement in and of itself.
For example:
Having air-bags in your car is a big deal.
It was meaningless to me, but when Jaime got the award, it was a big deal to her.
The opposite of a "big deal" is "no big deal". Of course.