Personality and luck obviously has some part to play in how 'easy' a baby is...but I disagree with lots of posts that seem to say this is the main factor.
Personally, I think how you parent a baby has just as much if not more to do with it.
Realistically, a baby that has been always held and carried around in a sling all day/rocked to sleep from newborn - yes, I think it's likely that this baby will find it more difficult to be alone/be put down (ie 'clingy' if you want to call it that...though I hate that word!) as they get older. A newborn that is always put down for a nap or spends more time not attached to a person - it's fairly logical to say that they may well be less likely to insist on being carried everywhere at 10 months as they've never been used to that.
Generally, I think there are also a lot of parents that like to use the 'naturally a bad sleeper/poor eater' as an excuse...not in every case, but in plenty.
My children's 'faults' - in lots of cases see how dh and I have 'caused' them if you like.
Examples - ds1 (age 8) is very, very competitive. Too much so, and tends to be a bad loser. When ds2 was young (ds1 was about 3, ds2 was 1), he had a bit of jealousy about the attention ds2 was getting. I made it into a game - 'Oh come on ds1, ds2 needs my help because he's just a baby - why don't you show me what a big boy you are and see how fast you can get dressed?'. It worked brilliantly at the time. With hindsight, i'm pretty sure the longterm outcome was to push him towards being too competitive and feeling like he needs to win all the time.
Both of my children are generally fantastic eaters - not fussy in the slightest. Both weaned the same way, all homemade grub bla bla. BUT - ds1 will choose a nice spagbol over a cake any day. He'll eat sweets and chocolate but knows when to stop. Ds2 however has a massive, massive sweet tooth. Doesn't know how to self regulate and would literally eat sweets or chocolate until he made himself sick if he had the chance. Ds1 (being a pfb) didn't even taste chocolate until about age 3. Never had a haribo or any sort of sweet until at least reception age. Ds2 however was eating chocolate buttons at 10 months and probably had sweets at age 2ish (because ds1 was having them by then...harder to refuse the second).
It's not hard to deduce that a sweet tooth in ds2 has likely been 'caused' by dh and me. Or that ds1's competitive nature has either been 'caused' or greatly exacerbated by my 'winning' games with him as a toddler. It would be easy for me to attribute both of these things to just their 'natural' personalities though, which i'm pretty certain isn't the case.