We are designing a website/web application where we hope to to achieve high user counts and, in general, lots of use. More specifically, we intend to be using PHP as the programming/scripting language and MySQL for the relational DB needs as a start. We have not yet figured out whether to use a NoSQL database or not.
Related to that, we want to design with scalability in mind. What are the most common scalability pitfalls for websites? What are the key areas that we need to take into consideration, so that the system can be easily scalable?