Picking a distro on "Provision" tab is very inconvenient.
The distros page is much better - at least has a search capability. But it lacks e.g. the ability to pick family and tag from list.

Could we use a (dynamic AJAX?) tree for easier navigation?

May be AJAX would be just an overhead and we could easily cache the whole tree - creating and keeping a separate JS file on server, which would be cached with expiration set to 6 hours or so? This could save CPU and bandwidth as people often access the tree again and again. The file would be regenerated when distro is added/removed/tagged.

/ breed / OS Major / OS Minor / arch / variant / tags

Also showing some marks for tags with name would be advantage.

This would be useful in Reserve page as well.

Sci-Fi?

An advanced graphics:

redhat
+ RHEL3
+ RHEL4
+ RHEL5
+ RHEL6
  + 6.0
    + i386
    + x86_64
      + Server
        + ALL
          + 2010
          + 2011
            + 2011/01
            + 2011/02
            + 2011/03
              + *S* StableDistro1
              + *I* InstallableDistro2
              +     OtherDistro
        + RELEASED
        + STABLE
        + INSTALLS
      + Client
  + 6.1
fedora

Some examples of shiny AJAXy trees here:

  http://www.dreamcss.com/2009/07/9-useful-javascript-and-ajax-tree-menus.html