First, why people chose Linux?
I think because it is free (of charge in the beginning). There is a learn curve for Windows too, you still need to do lots of tweaks if you really need to use it at full power. Linux too. So I wouldn't talk about the costs in this matter.
Then, it has the source open. Here comes the real freedom. At least I know that somebody will discover if some app will spy on you. You never know what kind of info Windows send to the mother ship and when.
Not to talk about the opportunity to adapt the code...
And, not at least, come the things that could not do with Windows.
Secondly, why people would chose Mageia?
Now, the followers count to see the Mandriva's quality in a better way of communication. I think the Mandriva's distro is the base in all aspects.
After that, it is place to improvement. Who knows, maybe Mageia will have its own life, far from Mandriva's path, but at same quality at least.
Me, personally, from the usability point of use, I would like it close as it is now in terms of available packages. I would like to have the opportunity to poll for some apps (that was the Mandriva Club members privilege afaik). I would like to have one DVD with all the stuff and an installer to allow me to install a server, a desktop, or to start a "live CD" environment. And a second DVD with the sources. Different choices in different settings, but same packages. I hate the idea of having several CD/DVD flavors (of course, to be available as many users would like that). I would focus on 64 bit (I have 64 bit cpu since the first one appeared on the market, now nobody does 32 bit cpus, even for multimedia, afaik). I would like to have some visual improvement on the desktop and a lighter KDE (but that's KDE team's job). I would like to have the KDE as classy as Gnome, but with KDE's plugability.
What I think it is a must in order to achieve success, as in Ubuntu's case, is a good collection of How to's. Whatever I wanted to do to setup something, tons of how to's for Ubuntu arose first.