Frankly, if you want a higher quality render output, it's not so much the models that need to be changed. Rendering lots of polys is still hard and a waste of time. The client engine really needs to be written to include some form of modern
bump mapping such as
parallax or
normal mapping so that more complicated geometry can be simulated without significant increases on hardware requirements.
Displacement mapping would be really nice. Solving graphics ugliness by increasing game model detail is the brute force method and isn't really as effective as other, cheaper (in terms of hardware load) solutions.
Also, none of these improvements are going to look really good unless and until we get dynamic lighting and soft shadows.
'High Definition' is marketing bullshit. As has been noted already, its only real technical meaning has to do with display resolution. When used as a descriptor in any other context it's meaningless (do commercials for 'HD audio' bother anyone else?).