Non-trivial development in WordPress requires a measure of internal knowlege about how WordPress works and how one can best work arounds it shortfalls in a professional large-scale and mission-critical/enterprise setting. Contrary to some rumours this isn’t more difficult than with other systems.
Despite initial impressions of a sub-standard data model and some historically grown design decisions, working around or even with “The WordPress Way” can be a very good strategy for delivering professional large-scale high-availability solutions. However, to achieve this, it is important to actually know how WordPress does things. Especially the things WordPress does particularly well and better than other content management systems. Here is a list of links into the official documentation of WordPress that are good and recomended “diving spots” for getting to know the professional ins and outs of the system and its principles. Recommended reading before you plan on reinventing the wheel once again.