Before I had more than one child, I frequently asked parents of more than one child (especially moms) how their kids were different. Usually the parents could give one or two details, but inevitably the answer came, "They're just different." It's something that I can totally understand now.
Here are some differences in my two boys:
Ike was usually big for his age, and Felix is barely on the charts.
Ike loved everyone, and Felix is more discriminating.
Ike liked to get in the mix and play with big kids; Felix is more likely to stand on the sidelines and watch before jumping in.
Ike never cuddled as a baby (he does now); Felix cuddles daily. (It's so nice!)
Ike was really independent in practically every way; Felix is just not. He loves to be held and generally requires more attention than Ike did as a baby.
Ike was somewhat oblivious to other people's emotions. (He's more aware now that he's older, but as a baby/toddler emotions were not on his radar.) Felix notices when people are sad and takes it very personally if he's chastised.
Ike wasn't especially picky as a toddler (though he is somewhat picky now); Felix is incredibly picky. This probably accounts, in part, for his small size.
Ike didn't want to stop nursing; Felix didn't seem to notice or care when I weaned him.
It took a lot of effort to make Ike laugh as a baby. (Again, his behavior as a little boy is different. He laughs readily now.) Felix has always laughed readily. He seems to be constantly on the lookout for something to laugh about.
All in all, though, they're just different.