i3 = If your into text based setting with no programming for setup and sane defaults.
There's a scale of how manage-y window managers are. Oldschool floating WMs are very manual in that you have to position and size each window on your own. i3 improves a lot on that by offering some automation, as long as the user still provides the basic idea of where to split and so on. Other WMs like xmonad, dwm et al. automatically position and size windows according to pre-defined layouts, which can be tweaked dynamically.
i3 resizing - meta + r and arrows
i3 floating - meta + shift + space move meta + left mouse button
An additional criterion actually is license (somewhat). I for one am put off by anything published under GPLv3, think it's nice i3 is released under the BSD-License. Definitely something I personally regard as positive.