* Write array literals with one item per line. This makes diffs which add or remove items far easier to interpret, and makes merging such changes feasible. And it looks nicer too.
* Use line breaks to show the logical structure of your code. This enhances readability. Bring similar elements in a list into alignment, in order to reveal the differences between those elements at a glance.
* Removed a fun game of spot-the-difference in AbuseFilterHistoryPager::getQueryInfo(). If I want fun games I'll play UFO:AI.
* Moved some oddly placed assignments (in expressions) to their own statements: such assignments reduce readbility.
-- Introduce tagging of individual changes (revisions, logs, and on recentchanges). The tags are customisable, and currently settable by the Abuse Filter and by the TorBlock extension. The tags can be styled on the various pages on which they appear.
-- Introduces a schema change, three new tables (valid_tag, change_tag, and tag_summary).
For MySQL 4.0 compatibility, store a summary of actions in the abuse_filter table.
While I'm there, make the pseudo-internal names for actions localisable and therefore actually make sense to the average user.