This note is primarily for those that build
against the Eclipse Platform's composite repository at .../eclipse/updates/4.3milestones
after the Platform milestone is delivered. While that's not a recommend
practice, I get the impression lots of people do it. For everyone else,
you will just find this information very interesting, I'm sure, but it
will not effect you directly.
Traditionally, we leave each milestone
"connected" to the composite. This allows rolling back an installation.
But, there has been so many changes since M5, due to the build technology
used and other changes since Juno SR2, that we plan to provide only M6
at that composite site, for M6. This will help "force" issues
to appear, if there are any, rather than waiting until the end of the release
and then everyone being surprised. Many details are in bug 395030 
- Disconnect old milestone repos for M6
But, I'll summarize here:
There are 6 or 12 changes related to
things we intentionally removed, that would be a factor regardless of the
build technology used (such as the "org.junit4" change many of
you already discovered).
There are 6 or 12 known issues related
to things missing from the new build that we plan to correct by M7 (mostly
missing *.source bundles).
But, there are several hundred changes
related to the "low level" p2 IUs (tooling* and *_root IUs) that
are involved in building products or RCP apps for specific platforms or
Operating Systems. We are assuming that the exact names of these IUs don't
really matter to anyone, and that everything needed really is there and
working, just named differently ... but ... that's the reason we plan on
forcing the issue by providing only M6 at that composite repo. More details
are in the bug so you can read ahead of time, but let us know, in that
bug or a new one, if there are issues encountered. We will of course leave
the older repos (M1 to M5a) around in case an "emergency work around"
is needed for the milestone, but we'll cross that bridge if or when we
get to it. We expect things to go smoothly, we just want to make sure.