- Patch #201122 by c960657, Moshe Weitzman: never write anonymous sessions, unless something has been written to . This is an important performance improvements -- as long as you use modules that use  carefully. It might be good to report some analytics about this in the performance settings page so administrators can see if there is a 'broken' module.

- Patch #333143 by justinrandell: cleaning up the Drupal bootstrap code, removing bootstrap flag in system table.

#305653 by Dave Reid, cdale, and snowball43: Fix bug where enabled themes get unmarked during update.php.

- Patch #309821 by Rob Loach: automatically put sites in maintenance mode when updates are being run.

#286035 follow-up by maartenvg: Fix problem with contrib updates not firing unless system update exists.

- Patch #252013 by Eaton, pwolanin, Susurrus et al: drupal_render() now printes #markup, not #value.

- Patch #245115 by kkaefer, John Morahan, JohnAlbin et al: after a long discussion we've decided to make the concatenation operator consistent with the other operators.

#208602 follow up by myself and webernet: invert conditional to properly update schema versions, when needed

#208602 by KarenS: add support for aborting all updates of one module, when a critical error happens

#200674 by scor, catch: update.php should inform users if their memory limit will possibly result in a WSOD

#199946 by JirkaRybka: append a short query string to CSS and JS files, changing on upgrades, so on core/module/theme upgrades, browser caches will 'flush'

#197722 by catch, hwsong3i: remove 4.7 to 5.x updates; we only support direct updates from 5.x to 6.x

#196630 by yched: postpone warning message display until batch finished, so messages are not forgotten on non-JS batches

#199336 by JirkaRybka: remove improper t() use in update code (update goes all in English, but there were some mistakes), and add messages to updates where there was no return value otherwise, so users know what happens in each update

#194310 by chx, catch, KarenS: run updates for disabled but previously installed modules, if they are compatible with the current system

#200271 by catch: more old 4.6 to 4.7 update path functions removed, as we don't support direct upgrades from 4.6 to 6.x definitely

 - whitespaces at end of lines
 - indentation
 - control structure usage
 - whitespace in empty lines
 - phpdoc comment formatting

#141727 by merlinofchaos, dvessel, sun: restore themeability support for maintenance pages (regression)

- Patch #179143 by Gabor, JirkaRybka, chx, ChrisKennedy, et al: do not fire bootstrap hooks during update.

#181758 by webernet: move cache_block creation to update_fix_d6_requirements() to avoid warnings about the nonexistent table through the update process

#176805 by dww: conditional linking to dblog pages in update.php, so if the module is not enabled, we don't get broken links

#162432 by bjaspan and chx: serial columns need their keys defined in db_change_field() and friends in MySQL

#171646 by JirkaRybka: store version usage information for every source string, optimize caching and prune the initially unused strings