For me, the killer feature of i3wm is not how it lets me arrange windows on my screen, but how it lets me switch between them. With OSX, I have to maintain a mental stack of recently-used applications: If I'm in my terminal and I want to get to firefox, I have to hit cmd-tab, or maybe cmd-tab-tab, or maybe cmd-tab-tab-tab. At least once a day, I make an off-by-one error and get stuck in a context-shift loop until I pause, think, and do the needful. With i3wm, I have 10 workspaces, and each is available with a single chord press. I have my own conventions about what's where, and there's no cognitive load associated with switching between text editor and documentation, or whatever.
Along the same lines, I like to keep a roguelike open for quick breaks, but ideally I'd never accidentally switch to a video game. OSX gives you no ability to punt an application from your cmd-tab list except to quit the application. With i3wm, I keep that on workspace 0, and have never accidentally switched over to it.
You may think I'm being precious, but it really does make a huge difference in my ability to stay in the zone.