/ must be on a partition or LV that will be formatted. Reusing an existing / is not allowed.

[Kamil Paral]

> > I imagine this could be very inconvenient for me when switching
> > distributions. I always have / and /home on the same partition. If
> > I
> > want hypothetically to go from let's say Ubuntu to Fedora, I delete
> > all files except /home and then select that partition for /. If you
> > force me to format that partition then I can't easily switch to
> > Fedora, because my /home is huge and I have no external space to
> > back
> > it up.
> 
> Why not have /home as a separate partition? This is the classic
> reason
> for doing so. I mean, what you're doing in the above is basically
> 'faking' a /home partition.
> --
> Adam Williamson

Because I never saw reason to do so. Having / and /home on the same partition saves you from problems with insufficient disk space in one place and too much space in the second place.

The only concern I ever had for my layout is whether the next distro installer would be smart enough to let me skip formatting of the / partition. Therefore I'm not thrilled to see Anaconda go the other way.