A lot of packages show important information in postinst messages. Many of them say things like "If you just upgraded from version FOO, you need to do BAR to make sure things don't explode into a million tiny pieces.". This isn't all that helpful when I don't *know* what version of the package I just finished upgrading from, because the old version of a package is only shown at the start of the emerge in the list of proposed packages, and that's now on the far end of twenty thousand lines of scrollback! It would be nice if emerge were to output both the old *and* new versions of the package in question above a postinst message, like it currently outputs the new version.
Ideally, the ebuilds should be checking the previous version and using that to make conditional adjustments to messages shown by postinst. I'm not sure if it's a good idea to build this into the elog facilities given that you can easily query the desired information with a grep of /var/log/emerge.log. There are also genlop (from gentoolkit) and qlop (from portage-utils) for querying emerge.log.
Ah, OK, didn't think of looking in emerge.log (nitpick, genlop is actually its own package not part of gentoolkit). Still seems a little unfriendly compared to just displaying the version number at the end of the emerge, but doable. I guess best is if packages actually check the old version number and display messages only when appropriate; admittedly I haven't seen the kind of behaviour I described for a little while, so feel free to close WONTFIX if you prefer that solution.