Big things to note:
* MySQL 3.23.x is no longer supported.
+* PHP 4.1/4.2 is no longer supported.
== Changes since 1.5 ==
* (bug 3649) Remove obsolete, broken moveCustomMessages script
* (bug 3291) 'last' diff link for last history line when not at end
* Avoid numerous redundant latest-revision lookups in history
+* Ability to set the table name for external storage servers
+* (bug 3667) Add missing global in page move code
+* (bug 3761) Avoid deprecation warnings in Special:Import
+* (bug 2885) Remove unnecessary reference parameter which broke classic skin
+ talk notification on PHP 5.0.5
+* Fix Parser::unstrip on PHP 5.1.0RC4
+* (bug 3852) "Redirected from" link no longer obscured on double-redirects
+* (bug 3845) Update attribute.php for 1.5 schema
+* changed directory hierarchy in images/math/. System upgrades from old to
+ new hierarchy on the fly.
+* (bug 3487) Fix category edit preview with preview-on-bottom
+* (bug 918) Search index incorrectly joined words at == headings ==
+* (bug 3877) Render math images into temp directory, then move to hashed
+ subdir so you can render new math images and have them work
+* (bug 3797) Don't expand variables and sigs in comments
+* (bug 3666) Don't spew PHP warnings in prefs on unrecognized site language
+* (bug 2392) Fix Atom items content type, upgrade to Atom 1.0
+* Allow $wgFeedCacheTimeout of 0 to disable feed caching
+* Fix WebRequest::getRequestURL() to strip off the host bits squid prepends
+* Require POST for action=purge, to stop bots from purging the cache
+* (bug 3817) Use localized date formats in preferences; 'no preference' option
+ localizable as 'datedefault' message. Tweaked lots of languages files...
+* Added local message cache feature ($wgLocalMessageCache), to reduce bandwidth
+ requirements to the memcached server.
+* Forbid usernames that can be interpreted as titles with namespaces, as that
+ leads to hard-to-manage names.
+* (bug 2721) Regression: Use European number separators for vi: wikis
+* Allow parser cache on redirect targets
+* (bug 3562) for go search, try Caps-Variants-Broken-At-Non-Whitespace
+* (bug 2569) Use PATH_SEPARATOR instead of trying to guess based on
+ DIRECTORY_SEPARATOR (was wrong on NetWare)
+* Require PHP 4.3.2 or higher strictly now.
+* Run wikitext-escaping on plaintext sigs (no wiki markup, just name)
+* Check for unbalanced HTML tags on raw sigs (markup allowed, but show
+ a warning in prefs and use default sig if not balanced)
=== Caveats ===