“The Id field is required” validation message on Create; Id not set to [Required]

You can add the attribute:

 [Bind(Exclude = "Id")] 

on the parameter in method rather than the class, that way on create you can exclude it and on edit it will still be mandatory:

public ActionResult Create([Bind(Exclude = "Id")] User u)
{
    // will exclude for create
}

public ActionResult Edit(User u)
{
    // will require for edit
}