At a minimum, I would expect standard USB, headphone, sd card reader, standard HDMI, mini displayport.
You're going to have a hard time convincing me that an HDMI connector adds more than a few grams of weight. Thickness, maybe. But even then.
VGA and ethernet could be nice too, but wouldn't see much use in my case.
The battery could be removable (i.e. half internal half external, like the thinkpads).
Optical drive is obviously not much use in a laptop anymore and should be external. (this one actually adds weight and not just thickness)