The Need for Speed franchise's first and longest era was mostly focused on luxury, sport and exotic cars. The era included driving on exotic locations and roads, and was also known for the then-high-detailed cars with in-door cockpits.
The second era heavily diverted from the original roots. The second era is based on the illegal street racing scene in the United States, popularized by the The Fast and the Furious movie franchise. The games featured car customization, and a new feature, visual customization, which formed the basis on the rest of the games in this era.
The third era partially returned to the roots, also explored different types of racing game genres, borrowed elements from other games, such as Gran Turismo, and also made a remake from the popular first era game, Hot Pursuit.
The fourth era is the current one, so we'll just have to wait and see how it turns out...