1) the customization options (tiling WMs ftw)
2) the ease of replicating the production environment I work with
3) the package managers
4) the everything-is-an-ascii-stream metaphor and the ease of creating and using tiny, tiny executables (e.g. shell scripts, daemons, composable cli utils, etc)
5) the complete and total freedom to install and use whatever I want, whenever I want without ever having to pay for anything at all, and more importantly ever have to ever think about entering a product key or having to register anything.
6) the tiny, tiny performance requirements and extremely wide hardware support
7) the networking toolage (ssh, curl, wget, scp, rsync, etc being either already there or one apt-get away)
8) the general feeling that complicated things are abstracted away only if it's possible to do so. The fact that things break transparently, or at least more transparently than the competition. The fact that it's beneficial to Debian that failures and shortcomings be discussed publicly and at length rather than being held behind closed doors to maintain a credible commercial image.
I would really like it if some OS could re-implement the, as I said, everything-as-an-UTF8-stream w/ networking as a first-class citizen, the blank slate UI to be built from scratch (and a few shells), while shedding the legacy crud of teletype-era in-band signaling keycodes, the X window systems, and so on, which I admit is very, very unlikely as it implies re-implementing almost all of the stack, at once, in a backward-incompatible way, all to re-implement a model (the CLI) that is seen as a technical relic to almost everybody except me and Neal Stephenson.
Let's just say, then, that I will switch OS when someone will be offering a consistent, minimal and usable UI metaphor, while accomodating the motor tics I developed using Vim for the last few years.