Getting used to good grooming habits as babies is a must.
I wish I had known what a feral cat was, let alone how to socialize one.
Ophelia was/is and while she LOVES her daddy, she's not as fond of me because of mistakes I made with her.
I wish I had known how awful declawing is, and that it is NOT a routine surgery to remove the nail beds (I guess the vet tech thought "nail bed" and "bone" meant the same thing). I almost lost my baby boy, and I may never forgive myself for putting him through that.
I wish I had known how much patience kittens require. They are babies, and that's something that's so easy to forget. They need constant attention and direction, or they'll find it themselves (which usually means trouble! LOL)
I wish I had known that they would 100% consume my heart and home. They run it, I just live there, feed them, give them water, play with them, and get paid by being allowed to love them. It's a pretty good arrangement.