Functions pertaining to Real/Fake groups (home realm and instance/battleground-specific groups, respectively) are removed. Instead, group functions take an argument specifying which group you want to query:
LE_PARTY_CATEGORY_HOME will query information about your "real" group -- the group you were in on your Home realm, before entering any instance/battleground.
LE_PARTY_CATEGORY_INSTANCE will query information about your "fake" group -- the group created by the instance/battleground matching mechanism.
GetAchievementCriteriaInfo split into two separate functions:
NEW Frame:RegisterUnitEvent("event", "unit1"[, "unit2"]) - Triggers the widget's OnEvent handler when the specified event occurs for the specified units.
NEW Button:SetEnabled(enable) - Enables or disables the widget (available for any widget with Enable/Disable methods).
NEW Region:SetShown(show) - Shows or hides the widget (available for any widget with Show/Hide methods).
UIPanelButtonTemplate2 is renamed to UIPanelButtonTemplate; the old version of which is removed.
The minimap buttons MiniMapLFGFrame and MiniMapBattlefieldFrame is renamed/merged to QueueStatusMinimapButton.
ShapeshiftBarFrame is removed; druid shapeshifting UI is now handled by the StanceBarFrame.
Observed bugs and notable differences
UnitIsGroupAssistant - the new version of UnitIsRaidOfficer returns false if the player is leader. The old function returned true.
UnitInRange - when not in a party/raid, the new version of UnitInRange returns false for "player" and "pet". The old function returned true.
Texture:SetTexture - If the path passed into Texture:SetTexture(path) is invalid, the texture will display as solid green.
Texture:GetTexture - If the path passed into Texture:SetTexture(path) is invalid, Texture:GetTexture() will still return whatever was passed in. Previously, it would return nil if the texture path was invalid. It seems that it is no longer possible to determine if a texture is valid or not (useful for validating user input).
COMBAT_LOG_EVENT / ENVIRONMENTAL_DAMAGE - the environmental type reported from this event is now reporting non-localized proper case strings (e.g. "Falling") instead of non-localized capitalized strings (e.g. "FALLING").