Artwork and Visuals

Icons (added tons of icons to the Breeze theme)
Refined plus Systray icons to match the general theme
Native Breeze widget themes for Qt4 and Qt5
New Look & feel / Megathemes system settings module
Plasmoid alternatives switcher
New clipboard Plasmoid
System load viewer Plasmoid
Icon theme for dark color schemes
Human Interface Guidelines (HIG) restructuring to improve usability
Visual glitch fixes

Workspace Elements

plasma windowed, a method to run Plasmoids as applications, useful for example for Plasma Media Center
Notifications and running job display has been polished
Logout and Lock screens now supports keyboard navigation
kdeplasma-addons returns with many add-ons coming back

Newly Available Widgets

The following add-ons have been ported from and are in Plasma 5.1 compared to 5.0:

Sticky notes
Fuzzy clock,
Timer,
System load viewer,
Icon-only tasks ("Icontasks")
Two new animated wallpapers: Hunyango, Haenau
Unit conversion search plugin
Konsole profiles search plugin

Digital Clock and Calendar

Time zone support in Digital Clock
Calendar renders much faster and more memory-efficient
Date display in the clock

Folder

Significantly improved performance and memory usage in desktop mode.
Faster, more responsive drag-and-drop operations.
Greatly expanded context menus.
Support for the 'associated application' system is back.
Keyboard navigation improvements (support for Home and End keys, etc.).
Spatial keyboard navigation (arrow keys, etc.) for unsorted views.
Tooltips can be disabled.
Icon size can be changed.
Better guard against corrupt configuration.
Many bug fixes (a few bigger ones: graphical glitches with drag and drop on some

drivers addressed, sort by size was broken, anchored range selections on unsorted views misbehaved, drag-onto-to-launch for view-internal drag and drop crashed didn't work, ...).

Bug due to which the view context menu now works when Folder is a panel popup fixed.

Kickoff Application Menu

Support for screen readers and other accessibility (a11y) software.
Layout fixes
Performance improvements in favorites model
Application descriptions are shown by default
Various other visual and smaller interaction bugs fixed

Kicker Application Menu

Basic support for editing menu entries right from the context menu (name, icon, command, etc.).
Per-instance hiding of menu items.
Basic integration with Muon Discover / package managers via PackageKit-Qt (you can jump into them via the context menu).
Support for screen readers and other a11y software.
Various bug fixes (a few bigger ones: improved positioning, "Add to Panel" with multiple applet instances in different panels could pick the wrong panel, initial keyboard focus when invoked via shortcut wasn't on the search field, ...).

Task Managers

Icons-only Task Manager ("Icontasks") is back.
Improved lifecycle behavior for launchers and startup notifications (clean handoffs/replacing, less visually messy co-existence).
More reliable identity mapping between startup notifications and launchers and windows.

Baloo Search

A user-visible query language which can be accessed via the kioslave. Example - `baloosearch://?query=tag:Fire`. The query language can handle quite complex expressions. This will create a "magic folder" which will be updated in realtime
Better UTF-8 handling in Searching - We now take diacritic marks into account. See this blog post
Removable Media Storage - We now never index Network File Systems by default (it can be enabled by the user, though).
Better detection of frequently modified files. We delay indexing for them.
Detecting xattr changes - Baloo is now more of a file indexer and will listen to changes on xattr for files. This means any application can set the tags/ratings by just setting the xattr of the file. Applications no longer need to talk to Baloo.
Overall speed improvements
Many more tests in the File Indexers

KWin Compositor and Window Manager

General

use of libepoxy
use of atlas texture for window decorations in OpenGL compositor
support for EGL_EXT_platform_x11 and EGL_EXT_platform_wayland
support for GLX_INTEL_swap_event in glxbackend

Wayland Support

new binary kwin_wayland
kwin_wayland supports starting the X-Server it needs to connect to
Wayland (client) connection is held in a thread
Support for using fullscreen shell interface
Wayland related code split out into a new kwayland lib