They are not the most accurate of drivers of the ball, but that's not needed at Augusta. Precise ball-striking is a must on approach shots, however, as you have to position a shot to the green in a area that you will leave you an easy putt. That's why players like Nick Faldo and Ben Hogan have won three and two titles, respectively. If you look at the champions of The Masters, poor putters don't win. Being able to not three-putt is important.
Length is important.
Just look at Tiger Woods's victory in 1997. Being able to reach par 5s with short irons is a big advantage over players hitting into the greens with long irons. Last but not least, experience and wisdom is important. That's why Jack Nicklaus has six titles. Yes, of course, Tiger Woods.