I don't really get all the defense for this decision. If you don't care about usable cargo space, that's fine. But all the arguments thus far are nonsense.
This is NOT a compact car. It's well into mid-size sedan territory. Even if it was compact, there exist compacts with liftbacks.
It's irrelevant whether whatever other cars you cherry-pick to compare it to have liftbacks or not. Other vehicles in the Model S's "class" don't have liftbacks either. Yet the Model S still does. And I haven't heard any complaints about the Model S's liftback impeding rear seat headroom; unsurprising, since the presence or lack of a liftback has nothing to do with rear headroom.
The only reason not to have a liftback is to keep the glass roof as a single piece. Sure, it's nice aesthetically, but the utility gained by a liftback far outweighs concerns over having to separate the glass roof into two separate pieces. Not to mention those who choose a conventional roof get screwed out of a liftback for no reason whatsoever.
I'm not cancelling my preorder because I'm hoping that they simply weren't able to get the final liftback design ready in time for the unveiling or somesuch, and will have something to show later. But if they keep the crappy little trunk-hole, it's a really bafflingly bad decision in what is otherwise a wonderful vehicle.