I have to admit, when it comes to window managers, my taste is a bit peculiar. But I’m glad to see other people also share my taste. I think the simpler the wm is, the better. Here is why I think that way.
User-friendly wms tend to lose on productivity in the long run. If a user is starting to use a new wm, friendly features are very useful. For experienced users, those features are useless or not efficient enough. So friendliness not only should not be a priority, it should be avoided in the long run. The priority should be efficient controls, shortcuts that empower you to manage your software efficiently.
The memory footprint of wms like Gnome 3, Unity and KDE is too large. An obvious argument is that wms like i3 are ideal for old machines. But more than that, it gives an advantage even on modern systems. If your system has more memory than you ever use, then you are wasting resources. On the other hand, if your system often performs swapping, it probably means more memory will be useful.
So, given that even a modern system will sometimes perform swapping, using a lightweight wm instead of a heavy one will result in a faster system.
After you have mastered your lightweight wm, the advantages are huge. The translation between what you want and how to do it comes easily.
Below I have listed a few features of i3 that I like.
If you plug another monitor into your machine, which I often do, you can switch between workspaces on one monitor without changing the other. I use this feature on several occasions. One of them is when I want to work while watching a movie. I can set the external monitor to hold the video and switch between workspaces on my main monitor without interrupting the movie.
At first, transition effects are beautiful. I remember starting to use Compiz and configuring my desktops to look like cubes. Seeing the transition from one face to another was awesome, on the first day. After a while I had removed all the transitions, not only to make it lighter but also to make it look lighter.
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Nowadays I just want no transition effects at all. I see them as useless and a resource hog.
By default, the first GUI occupies the whole workspace. The second GUI you open, on the same workspace, will split the space in two (horizontally or vertically). Of course, you control various aspects of how the windows are distributed, but the important thing is that you minimize waste of space. You don’t resize windows with the mouse, which would be counterproductive.