783 Menu item titles and descriptions should now no longer be wrapped in t() calls.
603 use $form_state['values'] instead, which is where the values are stored after a submit
248 Use the new Schema API to create and drop tables rather than db_query()
138 The menu system has been completely over-hauled in 6.x.
116 A new schema API has been added in 6.x
107 The arguments to hook_help() have changed
101 The parameters for form validation and submission functions have changed to $form, &$form_state.
77 New syntax for .info files files requires core=6.x
67 Submit handlers use $form_state rather than returning urls
62 When labelling buttons, make it clear what the button does, "Submit" is too generic.
60  +-1: Translations moved from ./po to ./translations
50 hook no longer exists, use hook_form_alter() to swap your own validation handler
47 Parameters of watchdog() changed, you must remove calls to t()
40 replace $form['op'] with $form_state['values']['op']
38 Use menu_get_object() to get an object based on your path
38 New syntax for .info files, use dependencies[]
37 url() and l() arguments changed, if you have a lot of these use this conversion script
31 new hook_theme() function is required to register theme_ functions
31 hook_form_alter() parameters have changed
27 Parameters to hook_access() have changed
25 db_num_rows() has been deprecated
24 form_set_value() parameters have changed
23 admin/logs has been renamed to admin/reports, including all subpages.
20 Use new ip_address() function instead of $_SERVER['REMOTE_ADDR']
16 The "administer access control" permission has been renamed to "administer permissions", and the path 'admin/user/access' to 'admin/user/permissions'.
14 db_next_id() is deprecated.  Use db_last_insert_id() instead.
11 taxonomy_node_get_terms(), taxonomy_node_get_terms_by_vocabulary(), and taxonomy_node_delete() now take a full $node object, not just a nid (node id).
11 $form['#submit'] and $form['#validate'] no longer support custom parameters
10 $locale became $language
9 taxonomy_get_vocabulary() is deprecated.  Use taxonomy_vocabulary_load() instead.
9 node_access_rebuild() should not be called from hook_enable() or hook_disable() functions any more.
8 Validation for specific form elements now uses the #element_validate property
8 Parameters to drupal_mail() have changed
8 Changes to cache_set() parameter order
6 in confirm_submit, use $form_state['redirect'] instead of return (remember to change function prototype to use &$form_state) 
6 in confirm_form, use $form_state['redirect'] instead of return (remember to change function prototype to use &$form_state) 
6 hook_nodeapi('submit') has been replaced by op='presave'
5 Remove $row argument from db_result() method
5 #multistep is gone, use $form_state instead
4 theme_get_function() has been deprecated because of template theming; see theme_get_registry()
3 Parameters for file_save_upload() have changed
3 hook_link_alter() parameters have changed
3 file_check_upload() merged into file_save_upload()
2 The node/add/$type menu items are now auto-generated by the menu system in 6.x. You should not declare them in your menu hook. See node/add is now menu generated.
2 Parameters to user_authenticate have changed
2 Parameters to book_toc() have changed
2 Parameters to book_outline() have changed
2 Parameters to book_export() have changed
2 menu_set_location() is deprecated.  Use drupal_set_breadcrumb() to set a custom breadcrumb or menu_set_item() to set the current location in the menu tree.
2 In place of custom_url_rewrite(), use custom_url_rewrite_inbound() or custom_url_rewrite_outbound()
2 hook_mail_alter() parameters have changed
2 FormAPI's #base property is gone, manually populate the $form['#submit'], $form['#validate'], and $form['#theme'] elements with the proper function names
2 book_tree_recurse() is deprecated
2 book_location() is deprecated
1 The parameters for form prerender functions have changed to $form.
1 Parameters to book_prev() have changed
1 Parameters to book_next() have changed
1 Parameters to book_export_html() have changed
1 Parameters to book_admin_edit() have changed
1 Parameters of watchdog() changed, you use $severity WATCHDOG_DEBUG instead of $type 'debug'
1 Parameters for drupal_retrieve_form() have changed, add $form_state as a second argument
1 locale_refresh_cache() is deprecated.  Use cache_clear_all() instead.
1 format_plural() accepts replacements, you no longer need to use ()
1 Comment settings are now per node type.
1 cache_set() and cache_get() automatically (un)serialize complex data types
1 book_tree() is deprecated
1 book_toc_recurse() is deprecated
1 book_recurse() is deprecated
1 book_node_visitor_html_pre() is deprecated
1 book_node_visitor_html_post() is deprecated
1 book_location_down() is deprecated
1 book_insert() is deprecated
1 book_form() is deprecated
1 book_content() is deprecated
1 book_admin_orphan() is deprecated