Why replace Nautilus?
Of course, PCManFM is not as powerful as Nautilus or Konqueror. That’t not the goal of the project. The project is an attempt to implement a modern file manager with maximal speed and usability while still keeping minimal resource usage and dependencies. Some programs are extremely fast but oversimplified and the user experience is poor. Others are extremely powerful, trying to include everything possible, but inevitably, they became too heavy. PCManFM tries to find a good balance between performance and usability. While supporting many new features of modern file managers, the performance on low-end machines and restricted hardware is still quite reasonable. Desktop independence and tabbed-browsing are another two important features. Here are the concepts and design goals of PCManFM: http://pcmanfm.sourceforge.net/concept.