Starting with the obvious, experience is probably the number one with attention to detail being a close second in my opinion.
I have been asked this twice in the last month from people considering entering the industry. Of course, experience is impossible for those starting out so for those considering it I would suggest the ability to think in three dimensions and visualize potential shapes is a key skill to at least have an understanding of before you start.
Another key skill to have is the ability to problem solve. Most good joiners will tell you it is this ability which separates the run of the mill operative from the outstanding.
A good understanding of mathematics is necessary for your work to remain fluid, reaching for a calculator every five minutes in a workshop environment is not an ideal situation.
Lastly I would suggest a new operative should have a good level of self confidence, keen understudies often stimulate tutors which is of course mutually beneficial!
Of course this is based on my own experiences. Others may form different opinions of which, with a view to helping others, I would welcome hearing and post. I hope some may find this helpful.