Caches for wikis with table prefixes will be lost on upgrade, caches for wikis without table prefixes will be preserved. Custom cache keys in extensions can be migrated at leisure. Extensions which write to core cache keys should be migrated ASAP, as I have done with Special:Makesysop.
* Deleted DatabaseMysql.php, no longer necessary, database classes are autoloaded.
* Moved wfGetMimeMagic() to MimeMagic::singleton()
* Fixed a couple of __CLASS__.'::'.__FUNCTION__ things.
* Made lines from initialiseMessages() appear as list items during installation
* Moved the bulk of the localisation data from the Language*.php files to the Messages*.php files. Deleted most of the Languages*.php files.
* Introduced "stub global" framework to provide deferred initialisation of core modules.
* Removed placeholder values for $wgTitle and $wgArticle, these variables will now be null during the initialisation process, until they are set by index.php or another entry point.
* Added DBA cache type, for BDB-style caches.
* Removed custom date format functions, replacing them with a format string in the style of PHP's date(). Used string identifiers instead of integer identifiers, in both the language files and user preferences. Migration should be transparent in most cases.
* Simplified the initialisation API for LoadBalancer objects.
* Removed the broken altencoding feature.
* Moved default user options and toggles from Language to User. Language objects are still able to define default preference overrides and extra user toggles, via a slightly different interface.
* Don't include the date option in the parser cache rendering hash unless $wgUseDynamicDates is enabled.
* Merged LanguageUtf8 with Language. Removed LanguageUtf8.php.
* Removed inclusion of language files from the bottom of Language.php. This is now consistently done from Language::factory().
* Add the name of the executing maintenance script to the debug log. Start the profiler during maintenance scripts.
* Added "serialized" directory, for storing precompiled data in serialized form.
* remove require_once() throughout whole code, yet left in few places
* move global functions in HttpUtils, ProxyTools, Credits to class methods
* php5 only: __autoload() now used, combined with class->file map and require()
* move initialization of $wgValidSkinNames to Skin::getSkinNames()
* few more changes that will surely break stuff.
** Database constructor calling sequence, and Database::newFromParams()
** Including flags in the server initialisation structs
** Support for setting appropriate flags from legacy globals in Setup.php
** Moved some defines to Define.php so that they can be used in LocalSettings.php, most importantly the bit field constants
* Changes related to post-parse link colouring
** Turn the link cache back on when using it for updating the links table
** No longer need to call preFill() on page view
* Better synchronisation of slave servers
** Rearranged getConnection()
** System for beginning and committing transactions when multiple connections are open
** wfAbruptExit() commits transactions, wfErrorExit() does not. Various functions changed to use wfErrorAbort()
* Allowed reporting of database errors during deferred updates by moving them above output()
