Things like:
A job where you've taken a leave of absence
Owning a house with mortgage payments
A wife/fiancee in the UK
To a lesser degree:
Family left behind in the UK
A job offer for when you return.
They need to be convinced that you will not try to overstay your visa or go into hiding once you're in the US.
You need to be confident in telling them that you will really return to the UK because you intend to fly big jets for BA not some foreign company..bla bla bla.....
Have fun....it's not such a big deal since you're (probably) not an economic refugee from certain parts of the world.