Few points of my own:
1)It runs on "cheap enough" hardware, AND there's a laptop option.

Talking about PPC Mac hardware here, obviously.

2)Modern "basic use" works.

I can watch videos on mplayer (as long as resolution isn't too high), mp3's play without problems on ANR and web browsing works nicely with Odyssey. I don't generally need any "office programs", and I like it that way. If I did, there would be working online versions anyway.

3)Working development environment.

Sure, it's not Eclipse, but I don't WANT it to be eclipse
As a "text editor with syntax highlight", Scribble definitely doesn't need to envy similar "not fully bloated IDE" choices available on other platforms. gcc (and Makefiles) work the exact same way as they do on linux. Also C64 development (another hobby of mine) works nicely with CC65.

4)"Extra" software.

Sure, many would prefer latest AAA games, but I'm not that interested in those. ScummVM, MAME, snes9x and other emulators provide lots of games to play. Of course there are also some more interesting "special" programs, such as SoundBankster.

Of course the OS itself also comes with useful, less often needed software for audio cd ripping, cd/dvd burning, ftp access etc.

5)It's simple enough to understand.

Add line to user-startup, and it will be executed on startup. Drop a program to wbstartup, and it will be launched on startup. Booting f.ex. linux is much more complex process, and I can't say I know what happens at which points, and why. Linux works for me, though, and I don't NEED to understand everything. Of course this extends to many other things aswell.

6)It's "different enough".

Often there's this new f.ex. linux distro that everyone hypes. I can't see why I should get excited about it. I use linux daily, it works for me. I can't see why "just another distro" would be so much different. If I use linux daily at work (I did while I was employed), if I want "something different" at home, "just another linux distro" isn't good enough.

7)Configurable UI.

On many linux desktops, I hate it when something seemingly simple cannot be configured (especially gnome is getting worse and worse in this, on purpose...). With MUI many things can be (re)configured, but none has to be.

8)Familiar enough.

Being (mostly ex) Amiga user, many things just seem familiar.

9)Usable command line interface.

I'll be honest, I think linux is here more powerful. On the other hand, I feel like CLI is kept "crippled on purpose" on Microsoft Windows, so that most people would just use graphical user interface instead. Being better than windows and somewhat worse than linux is a nice achievement.

10)Regular enough updates.

Of course they could happen more often, but can't demand too much considering the size of development team & the fact most have jobs aswell. It's much easier to start yet another hobby operating system project than keep updating it.

11)Communication with development team members.

It's actually possible to ask questions & get replies.