Yes Demi did it first, and many others followed. I'm simply excited to see more diversity on magazine covers, including pregnancy shoots. Serena is a legit powerhouse and I love her gorgeous photos.
To be clear I don't have a problem with Serena's photos, I think they're very nice and quite restrained, I like that they haven't dressed the scene in an extravagant way. If I was going to write a formal art critique for a school paper or something there are a few things I would take issue with but as a magazine cover and as a representation of Serena it works very nicely.
I do have a bit of a problem with the celeb pregnant belly convention... mostly because it comes off as the female equivalent to a d*ck measuring contest... if I may be so crude. If you're uber famous and you're not going to add something different to the canon of naked pregnant celeb shots, I say don't bother.
It's a funny what makes a great photo. The thing about Demi Moore's image (and I'm sure that photo was motivated by ego more than most) is the way she's looking out of the frame, if she had been looking in any other direction with a different expression I'm not sure the photo would have worked as well as it did. There's a quiet defiance that comes across in the pose, tilt of the head (and timing of the photo) that's not present in a lot of celeb pregnant belly shots.