I may not be as extensive as the others, but I note the follow:
hatch date (if I have it)
parents (if I have it because I register my corn snakes)
purchase date
age
morph
base weight
genetics (if I know them and include all hets)
day they were fed and if they refused
the size, amount, type, and weight of their prey item
when they shed
their weights each day they eat (so it's all standardized)

Now, I haven't begun breeding yet, but I plan to document the following (I already have a chart made up):
For the mother and the father, I include their names, species, morphs, genetics, weights at the time of introduction, the date they were introduced/seperated, and the days I see copulation
For the clutches, I will be recording the clutch number (I plan on numbering them to keep track), day the clutch was laid, as well as how many eggs (fertile and slugs)
For the babies, I will be recording their hatch date, morph, weight and gender.

Then...all the babies will be put into my excel sheet so I can monitor their weight gain, sheds, measl, etc.