Testing is going to become increasingly important. This is because the size of codebases and the number of people who are working on them are increasing. As more people touch the core of an application it's important to have tests there so they don't break it.
MVC frameworks are a proven method of structuring an application. They allow for modularity which we should all strive for. By breaking our applications down to smaller applications we can untangle the mess of spaghetti code which leads to more readable code, more re-usable code and bugs that are easier to find.
There is a lot of information that is available on the internet about different frameworks and high level concepts for large applications. What I'm going to do is introduce you to these concepts by example using Google's closure tools.
Next post I'll give a brief introduction to what a large application needs, what the Closure tools can cover and other alternatives.