Well, of course Tenggren had a hand in the looks of Snow White and Pinocchio, so those posters have that going for it that the new posters won't have because most of the people who worked significantly on the films are dead. Eyvind Earle isn't alive, so he can't make a Sleeping Beauty poster, which, if he did, would surely be incredible.
Oh, and
Magic Mirror, yea, on model is the correct term, here's a
link or
two that care to agree, so you don't have to put that term in quotes since it's not opinion but fact that the characters in Tenggren's poster do not match the model sheets of the final, finished film.
And if you just put that in quotes because you were quoting me, well oopsiemybad.
And 271286 brings up that Beauty and the Beast poster that almost makes no sense. Once again in proves the sophisticated "collector" artwork insists throwing the characters in shadow is more grown-up, somehow.
I've also seen Aladdin posters that are extremely dark. I think the posters are actually previewing the movie to come. You call them the original theatrical posters just because they came first, which is technically correct, but it's odd that during the run of the films they had both dark and bright "kiddy" posters (remember the poster with Belle in pink and purple), so I would say the dark ones were previews, the bright ones where you actually see the characters more for an audience who wanted to know what they would be seeing instead of a glimpse.
And I'm serious here, I think those are posters only previewing the film, discuss it with me. Here, the preview poster is on the left, the theatrical one on the right:

Now, Beauty and the Beast had a re-release, but I know both those posters came out duing it's debut, so I'd say the first one is a mere sneak peek at the film to come. Same case with Aladdin, which had no re-release so those two posters definately came out during the same run.
Now, if you really wanna see a
really artistic Disney poster, here:
EDIT: This is the IMAX 2002 re-release poster, by the way:

Not only is it beautiful, and cleverly uses the Enchanted rose which is so essential and symbolic in the film, but it is very artistic and creative. Notice how the Beast's silhouette is formed by the dark, imposing shadows, which perfectly represents his dark personality and dark situation, while a red/pink rose petal forms Belle's silhouette, representing her delicacy, her beauty, and the life and vitality she brings to the Beast. She is the warm bright light that saves him from the cold dark shadows.
Covering all the characters in shadows doesn't do that.
If anything, putting the characters in the dark would make the most sense for Cinderella, since she and the prince met and fell in love in the dark, and probably for a reason (it turns them blue, a color that represents the calm and good specifically in the movie, if you'd notice what characters wear blue and when).