Most of the above list is actually supported by xmonad (and dwm, and awesome and ...). The two things that are really the defining characteristic of ion3 is that it is a user-defined layout (not a "dynamic" layout, a la xmonad), and that the clients are represented as a tree of windows, rather than e.g. a zipper or list of windows (which is how the nesting works).
xmonad and its cohort basically make the proposition that user-defined layouts are inefficient, and you can be more productive with predictable, automated tiling. They directly opposed ion3 in that regard.
Mouse, configuration, floating etc is all fairly standard everywhere.
xmonad and its cohort basically make the proposition that user-defined layouts are inefficient
Well, having worked with both I strongly disagree with that assumption.
Of course it's all a matter of preference but I believe there's a lot of "you don't know what you're missing when you never had it" going on. I've yet to observe an awesome|wmii|xmonad user who doesn't constantly get into fights with the WM because they can't just quickly resize or re-order a few frames in a truly predictable way.
I used ion2 for a few years. Now I've been using awesome for a few years. They're both nice. Overall, I think I prefer letting the wm arrange my windows. There are a few times where I have considered writing my own layouts, but that itch has never lasted long enough for me to scratch it.
It is too bad that ion is no longer maintained. Choice is good.
xmonad and its cohort basically make the proposition that user-defined layouts are inefficient, and you can be more productive with predictable, automated tiling. They directly opposed ion3 in that regard.
Mouse, configuration, floating etc is all fairly standard everywhere.