@nixCraft interest and thus knowledge about how the lower layers of their systems work - would be my biggest. We have chosen to hire programmers and educate them in that.. slowly.. as even with 5-10 year exp. devops people we found that they are lacking the 'mental strength' to actually learn this. Its hard - but its really necessary. Being a big team we try to make it easier than ever for everyone - by having exp. colleagues to talk to.