TL, DR:

• Existing ranking system is now on a per-map basis
• A new ranking system, called CS Rating, will be used for premier mode, with global and regional leaderboards
• Inferno has been added to the CS2 limited test
• CS2 invites will now go out to "as many eligible players as possible". To be eligible means that you have an active matchmaking rank, have prime status, and be in a region where the test is available.
• MR12 is now officially implemented in competitive play.

Full release notes:

[ PREMIER ]

• Added Premier mode
• Added a visible CS Rating and CS2 Leaderboards. For more information visit the CS2 Leaderboard FAQ
• Added CSTV for spectating Premier matches

[ SOUND ]

• Fixed a case where grenade throw sounds would play twice
• Added music preview button to music volume sliders
• Various adjustments to sound effects

[ WORKSHOP TOOLS ]

• Added Paint Kit enhancements to CS2 Item Editor. Paint Kit finishes can now support:
• Enabling custom normal maps and AO for all styles
• Optional pearlescent mask
• Optional standard PBR roughness texture
• Custom Paint-By-Numbers masks (enabling Paint-By-Number masks for Spray-Paint style)

[ MAPS ]

• Added Inferno to all available game modes
• Various bug fixes and tweaks for all shipped maps

[ MISC ]

• Fixed a bug with hostage rescue prediction
• Enabled first-person shadow for carried hostages
• Fixed water effects showing in the wrong place when a bullet hit the water after penetration
• Various fixes for weapon finishes
• Replaced "Secondary Fire Hold" setting with "Zoom Button Hold"
• Fixed "Disable all voice comms" button in scoreboard
• "Temporarily Disable Incoming Chat" now mutes voice chat
• Various CS:GO settings will be imported when launching the CS2 Limited Test
• Fixed a case of inconsistent crouching with Duck Mode: Toggle enabled
• Updated the invulnerability visual effect
• Adjusted 'sv_infinite_ammo 2' grenade behavior to match CS:GO
• Desert Eagle third-person muzzle flash smoke now dissipates faster
• Continued iterating on smoke shape
• Adjusted Competitive mode match length to 12-round halves

[ CS:GO ]

• Recent ranked matches in "Your Matches" menu will now show Competitive, Wingman, and Premier matches

[ GAME AUTHENTICATION CODES ]

• Access to match history granted to third-party websites will now cover Competitive, Wingman, and Premier matches
• GetNextMatchSharingCode API used by third-party websites must now supply a match sharing code that is no more than 1 month old to obtain the next match sharing code.