I haven't done one of these before here, but they've been popular in the past and are even useful in the future when found via search engines.

These changes can help you get an idea of the type of things Blizzard is doing internally.

Global API Functions: New
  • AntiAliasingSupported
  • BuyReagentBank
  • C_Garrison.CancelConstruction
  • C_Garrison.CastSpellOnFollower
  • C_Garrison.CloseGarrisonTradeskillNPC
  • C_Garrison.CloseRecruitmentNPC
  • C_Garrison.GetAvailableMissions
  • C_Garrison.GetAvailableRecruits
  • C_Garrison.GetBuffedFollowersForMission
  • C_Garrison.GetInProgressMissions
  • C_Garrison.GetLandingPageItems
  • C_Garrison.GetOwnedBuildingInfoAbbrev(plotInstanceID)
  • C_Garrison.GetPossibleFollowersForBuilding(plotInstanceID)
  • C_Garrison.GetRecruitmentPreferences
  • C_Garrison.GetSpecChangeCost
  • C_Garrison.IsUsingPartyGarrison
  • C_Garrison.RecruitFollower
  • C_Garrison.SetRecruitmentPreferences
  • C_Garrison.SetUsingPartyGarrison
  • C_LFGList.ApplyToGroup
  • C_LFGList.CancelApplication
  • C_LFGList.ClearSearchResults
  • C_LFGList.GetApplicationInfo
  • C_LFGList.GetApplications
  • C_LFGList.GetAvailableRoles
  • C_LFGList.GetSearchResultFriends
  • C_LFGList.GetSearchResultInfo
  • C_LFGList.GetSearchResults
  • C_LFGList.Search
  • C_ToyBox.ClearAllSourceTypesFiltered
  • C_ToyBox.FilterToys
  • C_ToyBox.GetFilterCollected
  • C_ToyBox.GetFilterUncollected
  • C_ToyBox.GetIsFavorite
  • C_ToyBox.GetNumFilteredToys
  • C_ToyBox.GetNumLearnedDisplayedToys
  • C_ToyBox.GetNumTotalDisplayedToys
  • C_ToyBox.GetNumToys
  • C_ToyBox.GetToyFromIndex
  • C_ToyBox.GetToyInfo
  • C_ToyBox.GetToyLink
  • C_ToyBox.IsSourceTypeFiltered
  • C_ToyBox.PickupToyBoxItem
  • C_ToyBox.SetAllSourceTypesFiltered
  • C_ToyBox.SetFilterCollected
  • C_ToyBox.SetFilterSourceType
  • C_ToyBox.SetFilterString
  • C_ToyBox.SetFilterUncollected
  • C_ToyBox.SetIsFavorite
  • DepositReagentBank
  • GetCurrentLevelDraenorTalent
  • GetLootInfo
  • GetReagentBankCost
  • GetSpellsForCharacterUpgradeTier
  • HasDraenorZoneAbility
  • HaveQuestData
  • IsArenaSkirmish
  • IsPlayerInMicroDungeon
  • IsQuestSequenced
  • IsReagentBankUnlocked
  • JoinSkirmish
  • MonumentChangeAppearanceToTrophyID(trophyID)
  • MonumentGetSelectedTrophyID
  • MonumentGetTrophyInfoByIndex
  • MonumentLoadSelectedTrophyID
  • MonumentRevertAppearanceToSaved
  • MonumentSaveSelection
  • ReagentBankButtonIDToInvSlotID(buttonID)
  • RequeueSkirmish
  • SortReagentBankBags

Global API Functions: Removed from 18179
  • BNGetFriendInviteInfoByAddon
  • C_Garrison.GetActivatableBuildings
  • C_Garrison.GetBuildingActivationInfo
  • C_Garrison.GetLandingPageMissions
  • C_Garrison.GetMissions
  • MonumentGetTrophyIDByIndex
  • MonumentReplaceWithTrophyID
  • RequestQuestData

Global API Functions: Removed from MoP
  • GetExistingLocales

FrameXML Functions: New
  • C_Timer.NewTicker
  • C_Timer.NewTimer
  • ReagentButtonInventorySlot
  • ShowQuestLog
  • ToggleWorldMap

Events: New
  • ACTIONBAR_SHOW_BOTTOMLEFT
  • CHARACTER_UPGRADE_SPELL_TIER_SET
  • CHAR_RESTORE_COMPLETE
  • GARRISON_HIDE_LANDING_PAGE
  • GARRISON_MISSION_BONUS_ROLL_LOOT
  • GARRISON_MONUMENT_SELECTED_TROPHY_ID_LOADED
  • GARRISON_RECRUITMENT_FOLLOWERS_REFRESHED
  • GARRISON_RECRUITMENT_NPC_CLOSED
  • GARRISON_RECRUITMENT_NPC_OPENED
  • GARRISON_RECRUIT_FOLLOWER_RESULT
  • GARRISON_SHOW_LANDING_PAGE
  • GARRISON_TRADESKILL_NPC_CLOSED
  • GARRISON_USE_PARTY_GARRISON_CHANGED
  • HEIRLOOMS_UPDATED
  • LFG_LIST_SEARCH_FAILED
  • LFG_LIST_SEARCH_RESULTS_RECEIVED
  • LFG_LIST_SEARCH_RESULT_UPDATED
  • LFG_ROLE_CHECK_DECLINED
  • LOOT_READY
  • PLAYERREAGENTBANKSLOTS_CHANGED
  • PLAYER_AVG_ITEM_LEVEL_UPDATE
  • QUESTLINE_UPDATE
  • QUEST_CHOICE_CLOSE
  • QUEST_TURNED_IN
  • REAGENTBANK_PURCHASED
  • REAGENTBANK_UPDATE
  • SCENARIO_COMPLETED
  • TOYS_UPDATED

Events: Removed from 18179
  • ASSIGN_FOLLOWER_TO_BUILDING_RESULT
  • CHAR_RESTORE_COMPLETE
  • QUESTLINE_UPDATE
  • REMOVE_FOLLOWER_FROM_BUILDING_RESULT

Events: Removed from MoP
  • PLAYER_AVG_ITEM_LEVEL_READY

Widget API: New
  • Cooldown:SetCooldownDuration
  • Cooldown:SetHideCountdownNumbers
  • GameTooltip:SetMountBySpellID
  • GameTooltip:SetToyByItemID
  • SimpleHTML:GetContentHeight

Widget API: Removed from 18179
  • GameTooltip:SetMountBySpellID
  • GameTooltip:SetToyByItemID

Widget Handlers: Removed from 18179
  • Browser OnButtonUpdate
  • Browser OnError
  • Browser OnExternalLink
  • EditBox OnArrowPressed
  • GameTooltip OnTooltipSetEquipmentSet
  • GameTooltip OnTooltipSetFrameStack

Lua Enums: New and Updated
  • [[LE_FRAME_TUTORIAL]]
    • _GARRISON_BUILDING = 9 (new)
    • _GARRISON_MISSION_LIST = 10 (new)
    • _GARRISON_MISSION_PAGE = 11 (new)
    • _GARRISON_LANDING = 12 (new)
  • [[LE_ITEM_QUALITY]] (new)
    • _POOR = 0
    • _COMMON = 1
    • _UNCOMMON = 2
    • _RARE = 3
    • _EPIC = 4
    • _LEGENDARY = 5
    • _HEIRLOOM = 7
  • [[LE_UNIT_STAT]] (new)
    • _STRENGTH = 1
    • _AGILITY = 2
    • _STAMINA = 3
    • _INTELLECT = 4
    • _SPIRIT = 5

Check out Patch 6.0.1 API Changes on Wowpedia for a full list of changes since Mists of Pandaria!