Well, I like a lot of the features, but I suspect my favorite feature would have to be cruise control for low speed running and control. That would be followed closely by the great sound and smoke effects of modern stuff, and then the lighting effects. Obviously, this all should come in a quality package that is reasonably faithful to the prototype. I don't consider myself a true rivet counter, but I do like good and accurate detailing on a locomotive.
Truthfully, it's hard to pin down one feature because a good execution of a model is a sum of it's parts. If one part is severely lacking, that may render the whole experience moot.
