In the realm of 35nm technology, it becomes possible to have thousands of IP blocks that need to communicate efficiently. Large-scale integration of these blocks onto a single chip makes the use of truly scalable Networks-on-Chips (NoC) communication architectures inevitable. This paper provides an overview of the outstanding research issues involved in designing application-specific NoC architectures by considering explicitly the level of customization envisioned in the communication architecture. For each category of approaches, we discuss the significance of the problem, provide a problem statement and survey the relevant solutions to date.