Full list of Patch Notes (from 2.3)

Tech tree:
Implementation of unique unit with purple legend entry, and purple boxes for all unique units. Make sure any unique unit is not visible by more than one civ.
Tech tree should show up properly at all scales on both the left and right side. There were several clipping bugs when the window was at a 4:3 aspect ratio.
Scrolling to the left or right in windowed mode should feel right. Before it would only scroll in some cases when you were well outside the window boundaries.
Scrolling should be disabled when you are close to the bottom of the window so it doesnłt interfere with resizing.
Scroll and close buttons are always centered now in any resolution/aspect ratio.

Gameplay
Maximum Selection Size
Increased from 40 (2.3) to 60 (2.5). Backed down from 200 in 2.4beta due to aforementioned reasons
Hotkeys
Reverted hotkey changes from 2.3 that caused incorrectly functioning shift + alt control group commands
You can now hotkey groups 0-9. (An added control group over the original 1-9)
The hotkey reset button will not scroll the menu to the top of the screen anymore
Clicking reset hotkeys will restore the key to the default instead of making there be no hotkey for the item.

Localization
English strings are no longer locked inside language.dll, but use language_base.txt. Other languages will be converted to this new system in upcoming patches.
Steam Stats / Achievements
Single Battle Buildings Razed stat and Single Battle Buildings Lost stat was incorrectly using units lost and has been corrected. These stats/achievements will be reset on the official 2.5 release

Cheats
From 2.3, cheats for wood, food, gold, and stone are now 10,000 instead of 1,000 and should reduce the wear and tear on your ctrl + v keys.
Lobby / Quickmatch

Quickmatch should match players with the closest applicable ELO ratings and should use DM or RM ratings depending on which mode is selected.
You can now adjust the speed of quick match games. It was getting stuck on Normal whenever you readied yourself.
Lobby games are now sorted properly, with games on top reported closest to host according to Steam.
Regional settings for lobbies will be primarily based on your Steam download region settings, thus unsurprisingly it is recommended to select the download location closest to you.
You can filter lobbies and quickmatch by regional settings including:
Worldwide Shows everyone on the patch
Far Within a continent or two roughly.
Regional Your region and surrounding regions
Close Only games in the same region as you
Additional information in the lobby browser. Clicking once on a game will display added info including:
Names of players in game
ELO of players in game for the associated gametype
Indication of whether the game is RM or DM
Other misc information (i.e. scenario name)
Double click to join a game lobby

Performance changes from 2.3
Internal frame rate is capped at 60 (from 20).
User interface updates 10 times per second as opposed to once per second. (mini-map, progress bars, etc)
Introduced pixel smooth scrolling over “half-tile" scrolling. You may want to adjust your scrolling speed options if you find it is too slow.
Reduced “Command Lag". This is new to the final 2.5 build. Previous builds (including 2.5beta) presented an optimized terrain rendering issue any time a building completed construction or a farm was placed. Previously, it would trigger a full regeneration of the mapÅ‚s 3D terrain. As a result, this was introducing a significant “command lag" and undesirable performance tax. This was especially prevalent with rapid placement of farms. The official 2.5 patch has resolved the issue by regenerating only the affected terrain instead of the entire map.
If you encounter any issues with frame rate unlocks, you can revert to the original behavior by running the game with -ORIGFPS. You can right click on Age of Empires II: HD Edition in Steam, go to properties. Then click on Set Launch Options and enter -ORIGFPS. It will remember this setting, so you may eventually want to remove it later

Leaderboards & ELO Ratings
The ELO calculations have been completely rewritten from the live version in 2.3 and follow the old MS Zone calculations. These boards will be reset again at the go-live for 2.5. Here are the known rules
Leaderboard ELO calculations are only considered for games that contain exactly two teams and no AI players. Any other combinations of teams are not considered for ELO calculations, such as a 3 player free for all
There are two separate ranked leaderboards which encompass the following:
Deathmatch games will be ranked on the DM leaderboard.
Random Map, Turbo RM, King of the Hill, Regicide, Defend the Wonder, and Wonder Race are part of the RM leaderboard.
Any other mode will not be considered for ELO score calculations and is unranked, such as custom scenarios.
There is a minimum time that a ranked game requires before the game will be officially calculated on the leaderboard. 2 minutes for DM and 5 minutes for RM. Games under those time limits are considered pre-maturely completed and are not scored.
A pre-game penalty is deducted for starting any ranked match. The 3 points is refunded back at the end of the game whether you win, lose, resign, or quit, irregardless of the time limits for determining whether an
ELO calculation kicks in (5 minutes RM, 2 minutes DM). It is designed to penalize players that pull the network plug to avoid finishing games.
Any MP game that is saved and restored later will be considered an unranked game to avoid exploitation.
We are now tracking additional stats per leaderboard that will detect players that exploit disconnections. This information is accessible now to 3rd party websites, and may be displayed in future patches.
Crash Fixes
We have fixed a random crash affecting Windows XP users involving issuing move orders
which could cause games to occasionally crash during gameplay.

Known Issues
It is highly recommended when moving to the beta build (or when upgrading between builds) to create a new player profile to avoid any potential conflicts during the upgrade. You can do so within the game by going to options, deleting the current profile, and creating a new one.
If you do a fresh install of AOE II HD after having the game on your system, in certain circumstances text may appear to vanish from in-game. This is a result of font files in the steam folder needing to be read on startup. (Windows thinks you got rid of the fonts and didnłt initialize them again when you re-downloaded the game). Simply restart your machine and the text will re-appear.
We are tracking several rare crashes across multiple versions on windows XP during campaign play and will continue to investigate to improve stability.
For some players the steam overlay may present a significant framerate drop when visible. WeÅ‚re looking into this, but if you wish to disable the overlay in the gameÅ‚s properties simply uncheck “enable steam community in game".
Please ensure you are using a copy legitimately purchased through official retail channels (Amazon, Steam, GameStop, etc) and downloaded through Steam when attempting to play AOE II HD. We cannot support issues that may crop up with other versions.