1) You can control it from the shell (and hence scripts and hotkeys). For example, `i3 "move to workspace prev"` moves the window to the previous workspace.
2) You can obtain a lot of information of the window manager state (all windows, their sizes, their tree structure, the marks, etc) from i3-msg in JSON format.
1 and 2 mean that the WM is very highly programmable via the programming language of your choice.
3) Workspaces are not fixed. Empty workspaces don't exist unless you're currently in them. You can move a window to a workspace of any name and it will get created. Empty a workspace and when you move away from it, it will be destroyed. This makes it very convenient to work with temporary workspaces.
4) Windows are arranged in a tree structure. Normal windows are leaves. Containers are the branches that take you to those leaves. Containers can be in 4 modes: vertical, horizontal, stacked, or tabbed. That last one means you can tab any set of windows. Why do so many applications implement tabbing when it should be the window manager's job? Stacked is very cool in that it's like tabbing, but the window title doesn't shrink with each added window you have in them. Anyway, this point means that you get a lot of flexibility in how you organize your windows.
5) The configuration gives you a lot of control. These are many small things, so I won't list them all, but as an example, I can put a colored prefix on window and container titles to remind me what they're about without having to focus on them and see their content. I can also match new windows by some criteria and have them appear in a container I tagged without them gaining focus. This is very useful for when I'm doing something in the shell that will cause a window (or multiple) to repeatedly appear but I don't want to lose focus from the shell and I don't want the new window(s) to appear on top of it. As an example of this, I may be running selenium tests which could cause a browser window to appear to show how the tests run. I may also be doing some ad-hoc statistics in octave (cli) and have graph windows appear.