"Sweet Skills" are things that you couldn't do before you entered the mission field that you can now do ONLY because you entered the mission field.
For example: I can look at a number on a piece of paper and 'see' a totally different number. 1000 is NOT 1000, it's 200! Ask me about the Haitian currency some day.
Another example: I can tell Elijah to sit still in 3 languages. When Cecilia was growing up, she only had the privilege of two.