This is because your model is crossing the standard planes and by default the top plane is set as an opaque ground in the scene properties.
This not a big issue.
1- If you go to the Model tab on the right of the interface, you can select your model in the model set list
2-in the properties of this model, you will find all you need to rotate the model to your needs and snap it to the floor.
You can also use the center command if the model is to far from the world origin, and the Autosize command to fit its size to the world scale.
Please note that the model have to fit in the environment sphere (its size can be set in the Scene properties).
Thanks. I was thinking my model was bad for a while!
Christophe is correct.
I assume this means you had "Apply Auto-Size" option OFF in the import dialog. Having it turned ON, results in the geometry automatically snapping to the ground plane during import. Arguably, it should auto snap even when OFF...not sure if there's a reason it doesn't. And it would be better if the environment was resized to the model, rather than the model resizing to a fixed environment size. Hopefully both things that can be improved in the future...