So many, many things.
Their smell. Mmmm - warm, milky.
Their smiles and laughs. Especially the smile they give you when they see your face in the morning.
Their gorgeous silky, kissable skin.
Their expressive faces.
Their fascination with everything.
The indescribably amazing feeling of their tiny little hands. Smooth, warm, soft, yet firm. Holding on to your finger, or resting on your breast as you feed them. I'm cosleeping with my DD (nearly 25 weeks), and I often stroke her hand, or place my finger in it while she sleeps. I'm completely addicted to the feeling. I'm trying desperately to find the right words to describe it, so that years from now I can read them and remember how it felt.
Good luck with the rest of your pregnancy.
Everyone labours differently - maybe it won't be as bad as you think. But at least it can't last forever!
And who knows - maybe you'll have a good sleeper. And even if you don't, you'll suddenly find that you have all these reserves of energy and love that you never imagined you could draw on. I'm always amazed that, even when I think I can't go on, I find something deep inside to keep rocking my DD to sleep, or whatever.