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  et al.  Can you say 'registry'?  Drupal now maintains an internal registry of
  all functions or classes in the system, allowing it to lazy-load code files as
  needed (reducing the amount of code that must be parsed on each request). The
  list of included files is cached per menu callback for subsequent loading by
  the menu router. This way, a given page request will have all the code it needs
  but little else, minimizing time spent parsing unneeded code.

- 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.

- Patch #209240 by saxofaan, mike stewart, gopherspidey: fixed installation problem with multi-site setups.

#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'

#197720 by nedjo, scor, keith.smith, catch: inform installing users about PHP memory requirements of Drupal 6

#194369 by lots of contributors: move default files directory to sites/default/files which can be created automatically on install, so no need to bug the user about it, making the install process easier

Reported at http://groups.drupal.org/node/7843 by kkaefer: t() was used in install.php in place of st()

#197690 by JordanCronin (as GHOP 68): server permissions are now explained very well in a handbook page, link to that

#199874 by gaele, keith.smith and JirkaRybka: better explanation of site email address, slogan and mission based on usability feedback

#110981 by saxofaan: allow contrib modules in install profiles and runtime to have requirements, just like core modules; also fix a consistency between message presentation in install and runtime requirements

#189880 by keith.smith based on initial suggestions from O Govinda: much better help texts and descriptions for locale module, actually explaining its extended feature set in Drupal 6.

#152497 by JohnAlbin, bdragon, moshe weitzman, chx and myself: several user login tasks, such as session id regeneration were not performed in all cases, so centralize this

 - possible timeouts with installing/enabling lots of modules at once in core
 - enable install profiles to have more modules without fear of timeouts on install
 - bootstrap Drupal before each module load, so previously enabled modules are bootstrapped
 - let modules run their hook_requirements() (although actually calling them will be possibly fixed in another patch)

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

- Patch #163246 by keith smith, freso, O Govinda, catch, webchick et al: fixed minor spelling issues and fixed spacing issues.

#175876 by hazexp, webernet, keith.smith: unify module/theme/translation download links and include more pointers to these locations

- Patch #192242 by catch, quicksketch, webchick, keith et all: usability improvements for the vocabulary/term forms.

#189986 by keith.smith and bjaspan: reword description on install update notification checkbox to ensure people understand that this is a notification feature, not an automatic update feature

#100909 by webernet and RobRoy: improve cron error reporting by defining time tresholds, so if cron stops running after some time, it also becomes an error, but cron not run yet is not an error on installation

 - use a two pass localization process so localization is ready for the configure form and profile tasks
 - fix awkward form API workarounds which were introduced before we used a full bootstrap anyway
 - allow for more usable localized profiles by letting them skip language selection
 - lots of documentation improvements to profiles and the installer functions

#191310 by JirkaRybka, keith.smith: ship with a files directory by default and improve installation error messages checking for all requirements on the same screen, instead of putting the user through individual error screens

#190206 by myself: move misplaced Javascript codes from the form definition to where the form is output

- Patch #191073 by Keith: another good string cleanup by Keith.  (Also removed some trailing whitespace.)

#178581 by lot of contributors: opt-in for update module on install and update to avoid any privacy concerns