Big updates:
The major changes listed below each have their own topic in the prerelease forum.
+ Flexible interface engine (WALTER)
+ Take lane alignment
+ Take comping
+ Project Media/FX Bay
+ Media Explorer preview time selection
+ Media Item Properties dialog changes
+ Mouse modifier customization
+ Item/Envelope/MIDI editing and time selection behavior changes
+ Track input FX chains
+ Multichannel audio monitoring/recording
+ Track stereo width control and automation
+ ReaSurround

Medium updates:
+ FX: combined REAPER presets and VST patches/AU presets in a single dropdown
+ FX: user can save a preset as default, to be automatically set when inserting the FX
+ FX browser: option to display/change default preset without opening the FX
+ Project sample rate snap (enable in the snap settings dialog)
+ SRC: Improved sub-sample accuracy when used with project sample rate snap
+ Negative media item start offset support
Drag the left edge of unlooped media to pad the start of the item with silence
+ Track Manager: basic report-style dialog to manage track visibility and selection
+ Video playback performance improvements and fixes
+ Project time selection is visible in MIDI editor (separate from loop points)
New theme color and blend mode for MIDI editor time selection
+ Edit cursor can now be included in undo state (via option in Prefs/General)
+ Live FX multiprocessing now works more effectively when tracks are in folders
+ Multichannel item improvements: allow choosing any mono/stereo pair from multichannel items
+ Multichannel routing improvements: can send track channels to higher master/parent track channels
+ Most windows have an always-on-top (pin) button on the title bar
+ Support for up to 62/64 MIDI inputs/outputs
+ Floating toolbar can be docked over the main toolbar
+ Separate default settings for crossfade shapes and autocrossfade-on-split length
+ Track and send faders can be linked to MIDI volume/pan
+ Automute safety feature is a global setting (rather than per-project), enabled by default
+ Automute safety feature can be applied to all tracks
+ MIDI editor: options to display time positions as bars.beats.hundredths or bars.beats.MIDI_ticks
+ Media item selection sets can be named/saved/loaded using the screensets dialog
+ Configurable transition time for automatically created envelope edge points
Allows adjusting the ramp time when editing envelope segments
+ Draggable plugin sidechains
Drag track IO button to a plugin window to add destination track channels 3+4 and create a send

Small updates:
+ Splash screen now displays when unloading projects, useful to get status
+ Splash screen runs in separate thread on Windows, allowing more useful status display during loads
+ When loading projects, much of UI is dimmed to reflect its inactive state
+ Media item: fade fill now drawn with more accuracy at edges
+ Media item: options to prevent editing fade-in/fade-out when the media item is small
+ Track recording input: can be set to "no input"
+ Track VU meters: can be disabled, or set to show peaks from all track channels (not just 1+2)
+ Installer (Windows): added "portable install" checkbox and more command line options.
"Portable install" will install REAPER with no icons, registry keys, or uninstaller
User preferences and settings for portable installs are kept separate
+ Basic time and beat grid in Media Explorer
+ Actions to choose take or choose take comp for media item under mouse
+ Actions to fit items to time selection, either looping or padding with silence as needed
+ Actions to change or cycle through fade shapes for fade-ins, fade-outs, and crossfades
+ Action to interactively adjust crossfade time for any touching or overlapping media items
+ Action to toggle "run FX when stopped" (temporarily stop audio processing while adjusting a buggy plugin)
+ REX: action to explode imported REX container into slices
+ MIDI editor: event position and note length can be edited directly in event list view
+ MIDI editor: event list view does not display events that occur outside the visible part of the media item
+ API: added SnapToGrid function
+ API: added CountTCPFXParm, GetTCPFXParm functions (to access FX knobs the user has placed on the TCP)
+ More windows obey "cascade all floating windows" action (Preferences, MIDI filter and others)
+ Tab-to-transient/dynamic split: actions to set and adjust transient detection sensitivity
+ Tab-to-transient/dynamic split: option to always use zero crossings (avoids clicks but less precise)
+ Track IO, ENV buttons open dialogs to the right of the track panel rather than at the button
the second click closes the dialog rather than refocusing it
+ Menu items throughout REAPER will be grayed out when the action is not possible in the current context
+ OSX: support for (usually older) MIDI hardware that sends running status messages
+ Track names: preserve numbers at the end of track names when the track is narrow
+ Mouse click below the last track in the arrange view can optionally clear the media item selection

Changes to default behavior (most can be changed via preferences):
+ Take lanes are aligned by recording pass
+ Shift+alt drag adjusts media item pitch (Preferences/Editing Behavior/Mouse Modifiers)
+ Attaching track envelopes to media items creates envelope edge points (right-click toolbar envelope button)
+ Control+C and control+X are mapped to new actions to cut/copy within time selection if there is one
+ Moving and copying items/envelope points/MIDI notes respects the time selection if there is one
+ Envelope point selection follows time selection
+ In time signatures other than x/4, project grid is interpreted as note length rather than number of beats