By definition, rustic relates to the countryside or something plain and straightforward in fashion, so the elements that make up a rustic house plan are such. They include natural materials, such as wood and stone, and keep them as simple and organic as possible.
Modern rustic house plans play up the exposed natural elements of the home, such as wood beams or stone walls, and then enhance the organic nature of the home by focusing on earth tones in the paints and décor.
Yes, as mentioned above, it's easy to create a home that combines the warmth and earthiness of a rustic home with the chic feel of a modern home. Scandinavian homes are a perfect example of the blend of current rustic house plans, sometimes called hygge.
The rustic house plan has become popular because many people find the design to be a warm and inviting contrast to the stark, cold designs offered in many industrial or modern house plans today. Rustic homes have a lived-in feel, even if they've just been built, that makes people feel right at home.
Rustic home plans and farmhouse plans are pretty alike in many ways. Still, where rustic is less refined and leans toward neutral hues, farmhouse favors whitewashing and more sophisticated design elements.