Every single thing! Particularly all the baby / toddler milestones.
Sleeping - would he ever sleep through - is it wrong to rock to sleep - should I do control crying, was it wrong to co-sleep etc
Speech and language - was he delayed, not pronouncing words correctly, and then when it came to school - difficulties with reading / phonics
Potty trying / bed wetting (I actually joined MN for advice when DS was 6 and still he'd wetting!)
Weaning /early feeding /toddler feeding - would he grow up only living off breadsticks, cheesy pasta and potato waffles..... then at age 11 was I wrong to give him so much treat food to keep his weight up as he enjoyed healthy food, fruit, veg but was so active ...
I think that most children 'get there in the end' and most worrying is unnecessary but we still do it.
DS now 17, enjoying working on apprenticeship scheme, good friends, his first girlfriend, learning to drive ... but there will still be things to worry around e.g. when he gets his first car, travels to gigs, holidays etc I've just had to accept that if he chooses to come home at 2 am after a night out it's not doing my health any good to wait up- but I still feel relief when I wake at the sound of him coming back.