Once they were in preschool, mine were in daycare at least part time so I kind of had a clue I wasn't the only one and then I didn't worry so much. I found the baby years the hardest so by the time we reached toddlerhood, I just felt immense relief :)
We had some concerned regarding development (hearing and such) and so we had professionals involved. I guess our concerns were not likely typical. That was a stressful period (with one of our children) when I think back.
Sleeping by that age wasn't a concern. I had figured that out. I did have to get up early with a couple of mine and I just viewed that as a phase and part of parenting. It was short lived in the grand scheme of things.
Eating - mine were 'picky' for quite a few years (until school age) but I learned, like everything, to work around it. Pick your battles. Instead of viewing it as them being picky, they had certain things they liked over others. My kids still do to this day, but within their 'range' there were quite a few options. So I expanded on those. I just offered something most would like at mealtimes with a side or two they would also like. Life is too short to make meal times stressful. They are all now pretty good eaters - it happened naturally, over time.
I think what I wish I could go back and tell myself is - work with them, their natural sleep rhythms, their natural likes/dislikes, their personalities, etc. instead of trying to work against them. That would have saved me a lot of time and frustration those early years. I did finally figure it out. When I see moms with lots of kids, and they don't look harried but they look like they've figured it out - that's often why - they've had time to practice. They learned to go with the flow.
But food, toilet training (again, go with your child's own development), sleep, common behavioral issues (tantrums, etc.) ... are the common ones.
The book B mentions below was helpful in knowing what to expect/when.