The turbine nacelle with traditional wind power generation system is heavy, especially in offshore applications due to the large mass of the power frequency step-up-transformer operated at 50 or 60 Hz, and copper conductor generator. For example, the weight and volume of a 0.69/33 kV 2.6 MVA transformer are typically in the range of 6-8 t and 5-9 m3, respectively. The weight for a 10 MW direct drive permanent magnet generator is about 300 t. These penalties significantly increase the tower construction, and turbine installation and maintenance costs. The tower cost represents 26% of the total component cost of the turbine and on average about 20% of the capital costs are associated with installation. Typical maintenance cost of an offshore wind turbine is about 2.3 cents/kWh, which is 20% higher than that of an onshore based turbine. As alternative approaches to achieve a compact and lightweight offshore wind turbine nacelle, different concepts have been proposed in recent years, such as step-up-transformer-less system, medium-frequency (in the range of a few kHz to MHz) power transformer-based system, multilevel and modular matrix converter-based system and superconducting generator-based system. This paper aims to review the technical challenges, current research and developmental trends, and possible future directions of the research to reduce the weight and volume of the nacelle. In addition, a comprehensive review of traditional wind power generation technologies is conducted in this article as well.