Tempreture........ optimum tempretue varies througout the incubation period sometimes by as little 3f also alot of breeds need a slight cooling period once to twice a day, this is a little tricky as its the core temp that matters here not ambient air temp.
Humidity.......very very important that the correct humidity is maintained it plays a vital vole in development not just hatching rates but affects many aspects of the embryo.
lighting only seems to have a role in the last few days when eggs are moved from the incubator into the hatcher.
Turning again matters alot of rubish is spouted about only turning a odd number of times in a day and doing 3,5 or seven times. the reality is correct turning can have a large impact on defects at hatching and size of chick hatched.
candling is important and can affect many of the other aspects of incubation. a dead egg in an incubator is doing nothing but take up space,costing money and a potential source of bacteria
oxygen and carbon dioxide play very subtle roles in both vigor hatchability and one area thats little known is that they play a significant role in the sex ratio of the chicks. it is little understood and one area i am continuing to do the most research on.
then you have bacterial control, whole batches can be lost if the bacterial load is too high.
all theese things are being applied to chickens in this case but i am shareing research with the RSPB and london zoo, everything that i am trying to do can equaliy be applied to other rare bird species.
i agree with mike that on the face of it all seem easy to control and indeed they are if you want hatch rates and quality that equates to the norm. to use an anology it isnt that hard to design and build a normal car and it will get you from A -B adequately, but you cant build a F1 car the same way. tolerances matter.
Farming is changeing and indeed needs to we have to take a more scientific approach theese days in order to maximise yeilds, i would prefer to do it by working with nature and enhancing what a chicken does that go down the road of eggless hatching that some scientist are currently doing