Stability, with a dash of daring, and a very astute rugby mind.
Let's look first at the ability of Rennie to handle the pressures of the job, especially the off field areas where Michael Cheika, who down to the last daysof his tenure was having public arguments with his boss, Rugby Australia CEO Raelene Castle, was always a bad headline waiting to happen. Imagine, for example, how explosively Cheika might have reacted if he had basically been accused of cheating.
It actually happened to Rennie when he was coaching the Chiefs, and how he behaved is a good illustration of staying focused under fire.
During his last couple of years at the Chiefs, before he went to Glasgow in 2017, Rennie had a weekly chat on Radio Sport with D'arcy Waldergrave.
In April, 2016, there was huge controversy when the last five minutes of a desperately close game in Wellington between the Chiefs and the Hurricanes was won 28-27 by the Chiefs, with the last five minutes played under "golden oldie" scrum rules when Chiefs' prop Siate Tokolahi left the field injured, and the Chiefs said they didn't have another prop who could pack on the tighthead side.
A story later in the week in the Herald suggested "gamesmanship" and "manipulation" by Rennie, hinting that Tokolahi's injury may have been "very convenient" for the Chiefs, who were being hammered at scrumtime by the Canes.
When Waldergrave came to work on the Thursday colleagues immediately suggested this was one week Rennie wasn't going to be talking.
Waldergrave rang anyway. Rennie, he quickly discovered, was almost incandescent with rage. "I said, 'How are you Dave'? He said, 'I'm ****ed off Darcy'. 'Are you still going to talk with me this afternoon'? Dave said, 'Ab-so-bloody-lutely I am. I want to say exactly what went on, and why we did what we did. Because I've been accused of cheating, and that's not how I go about my business'".
Sure enough a few hours later Rennie was on air, containing his anger, instead clinically detailing how Tokolahi had been spotted by coaching staff on the sideline staggering before the decision to take him from the field. "He wasn't afraid to face it all head on", Waldergrave told me this week, "and to this day I've got a huge amount of respect for him for doing that, and for how he did it."
How daring is Rennie? When he was a coach still on the way up, back in 2007 in Palmerston North, running the Manawatu Turbos, he spotted an 18-year-old kid called Aaron Cruden, who was completing his first club season out of Palmerston North Boys' High School.
Rennie sat him down and delivered the sort of message every aspiring young player would love to hear. Five years later Cruden would tell me, "Dave said, 'I think if you put in a lot of hard work you can really make a career out of this.' It was a conversation that really clicked for me".
By the winter of 2008 Cruden was basically living a young footy player's dream, playing for Rennie's Turbos and making a powerful initial impression. Six games into the rep season everything changed.
Cruden was diagnosed with testicular cancer. "I was quite lucky that mine was caught relatively early. But it had spread to the lungs and that's why I had to have some chemotherapy. The doctors were really positive that with a high dose of chemotherapy they could kill it at the source, and that was what they did".
Chemotherapy is an amazing tool in modern cancer treatment but specialists will tell you that to deal with a disease as ugly as cancer, the treatment itself has to be ugly too. The drugs are so toxic nurses administering the medicine that's going directly into a patient's bloodstream wear sturdy rubber aprons and gloves to make sure nothing spills on their bare skin.
Cruden had terrific backup from his partner, his family, and his friends. As well as care and affection he had a dream too. As far fetched as it might have seemed while he was lying in his hospital bed, he was determined to have a crack at making the New Zealand under-20 side for the world championships in Japan, a team that was coached by Rennie.
Cruden says that after his treatment he "still had a couple of months to prove my fitness to Renns and the coaches, and I was lucky enough to be able to do that".
Fate had one more curve ball to throw his way. In the first minutes of the final trial for the World Cup team Cruden damaged a medial ligament. He was operated on immediately, and four weeks later was heading off to the under-20 World Cup as New Zealand captain. It was just 10 months since he had been diagnosed with cancer.
Rennie's belief in Cruden stayed rock solid. "Everything we'd seen of Aaron at school and club and provincial level showed he was able to fit into the next level with ease. He's the type of character you want involved in your group".
What was just as startlingly in hindsight was the steely trust in his own judgement Rennie's decision demonstrated. It was Rennie's first year in charge of a national side, and as a rookie coach at that level, if he'd got things wrong, it would have been a disastrous start at a higher level.
In fact, Cruden led his side to a magnificent tournament victory. In the 44-28 win in the final against England he scored two tries, and kicked three conversions and a penalty. Cruden was named the IRB junior player of the year. "His performance in the final was just mind boggling", says Rennie. "I'm not sure we'd have won it without him to be honest".
How astute is Rennie? Last year Sam Cane talked with me about how clever coaching can give a team the winning edge. You won't find anything about cleaning, or clearing out in a rule book. But a few years ago the IRB instructed referees to allow players without the ball to be knocked over if they were in the proximity of a breakdown.
The Chiefs' coaching group, headed by Rennie, picked up on the possibilities ahead of anyone else.
"In 2012 and 2013 our point of difference at the Chiefs (who won super titles in both those years) was the clear out", said Cane. "The coaches we had were very smart, and we would do what we could to get an advantage. We had a simple philosophy. If there were less people on their feet there was more space to attack".
****************************
As Jamie Joseph and Rennie dropped off the All Black candidates' list questions have been asked about why they weren't more closely pursued by New Zealand Rugby.
It's a fair question, but what exactly would the approach have been? "We'd like you to apply for the All Black coaching job, which has the greatest depth of home grown coaches on the candidate list of any country in the world? Please ignore the contracts being shown to you by Japan and Australia of a guaranteed position, and instead take your chances against one coach (Ian Foster) who was an assistant when the All Blacks won a World Cup in 2015, and another (Scott Robertson) who has just become the first person to win three Super Rugby titles in the row. We hope it goes well elsewhere if you miss out here."
So, if you were in Joseph or Rennie's shoes, would you be racing to book a ticket to Wellington for the interview?