Geometry:

No isolated vertices
No coincident vertices
No coincident/coplanar faces
Correct face normals
No empty objects

Topology:

Quads and Triangles only
Model can be cleanly subdividable

Other:

lighting and cameras are not included in the product (Model file includes only the 3D model)
Model Scale: real-world scale
Model centered at origin (0,0,0)
all objects are named and logically grouped