Large-scale data is a fact of life for businesses today. While NoSQL provides several viable solutions to handling big data, they will not replace relational databases, but rather add to them. Each data model has its particular use, and businesses will benefit from choosing the type of database suited to the nature of the problem they are trying to solve, keeping in mind that they can have multiple types of databases in use in any one organization. Relational databases are familiar, and can be used for utility projects, whereas NoSQL is an immature technology–very new, so not many people know how to work with it, but it may be worth the risk for a business to use NoSQL if they are implementing something strategic that needs to handle big data or have fast access for a competitive advantage (such as with Amazon).
Plugins are a big reason why Wordpress has gotten as powerful and as popular as it has, for they extend the functionality of Wordpress in many different ways, including adding shopping carts, boosting SEO ratings, adding widgets and calculators to make conversions, etc.
When you look at a webpage, you see it as one page, with various elements (masthead, footer, content area, sidebar, etc.), and in straight html, you code it as one page, with the DOCTYPE declaration at the top, opening and closing tags for various parts of the page. With themes, though, these parts are divvied up and each one becomes their own page, functioning under a model similar to the Model View Controller.