The main use for my Mac is to move words around. (Picomac represents a new way of doing that for me, since I'm moving sound in addition to text!) As I manipulate text, I need to translate seamlessly from word to word, line to line, window to window, and app to app.
A major difference between the Mac and iOS is the full modularity of apps — just about anything can talk to anything else, and if they can't do it directly, a third party can step in easily to make the connection. My duct tape of connecting apps and data is Keyboard Maestro. It fulfills very specific roles, from the esoteric to the mundane. Among the more common tasks I use it for is app switching.
My app switching style has been a gradual evolution since the classic Mac. In OS 8, I relied heavily on the tear-off app switcher palette — that was the first step beyond navigating to the application via the Finder and double clicking it. On my first OS X machine, I started using LaunchBar, and I continue to use it to this day (a Mac without LaunchBar feels lobotomized to me, even though Spotlight has made significant strides recently). It too does much more than launch apps, but that core task is in its name. Press command-space, type a few letters (even some anachronistic ones work, like "ADB" for Contacts), and you're there.
It's possible to get even faster than LaunchBar for one's most commonly used apps by having global hotkeys. Some apps, like TweetBot, can establish these within their own settings, but that leaves many apps (including first-party ones) out of the workflow. Prego, Maestro.
I've heard of app-launching schemes that use number or function keys for the user's "top ten" apps, but this requires memorization. Instead I use a mnemonic system with letters. The trick was to find a combination of modifiers that would never be interfered with by in-app shortcuts. I selected control+option and I've never run into difficulties. From there the mnemonic choices are what works for you: some of my shortcuts are transparent — ⌃⌥T for TweetBot, ⌃⌥F for Finder — while others are less obvious — ⌃⌥R for Preview, because ⌃⌥P was taken for Pages.
Keyboard Maestro can not only switch to an app, but perform actions within it. So when I switch to Airmail (with ⌃⌥E for "email"), if there are windows open, it leaves them be; but if no windows are open, it goes to my inbox. For Chrome or Safari (⌃⌥B for browser), I get a new tab so long as none exist. This augments the app-switching behavior beyond what command-tab or even LaunchBar can provide. And for less commonly used apps, I still have LaunchBar and, yes, even the Finder to fall back on.