In the past few decades, there has been explosive growth in the construction of nanoparticle-based drug delivery systems (NDDSs), namely nanomedicines, owing to their unique properties compared with traditional drug formulations. However, because of a variety of challenges, few nanomedicines are on sale in the market or undergoing clinical trial at present. Thus, it is essential to look back and re-evaluate what these NDDSs can really do in vivo, why nanomedicines are regarded as potential candidates for next-generation drugs, and what the future of nanomedicine is. Here, we focus mainly on the properties of NDDSs that extend blood circulation, enhance penetration into deep tumor tissue, enable controllable release of the payload into the cytoplasm, and overcome multi-drug resistance. We further discuss how to promote the translation of nanomedicines into reality. This review may help to identify the functions of NDDSs that are really necessary before they are designed and to reduce the gap between basic research and clinical application.