Description taken from the main forum found here:

The patch contains various bugfixes and game improvements such as widescreen support. It also enables the use of the FMSel FM loader frontend with Thief 3, and provides some GUI tweaks. It works with the GOG and Steam versions of the game.

Original game bugs and issues that have been fixed or improved:

Multicore fix prevents the intro video from looping
The City sections are now played on your chosen difficulty instead of always being on Normal
Frame rate limiting fixes broken lockpicking, ladder climbing and wall flattening
Some speed-up of level loading, especially with vsync on
Fix for the desktop being visible during level loading on Vista and up
The game will no longer crash when binding an analogue controller through the keybinds screen
Fix for controller axes sometimes being set up reverted

Game improvements:

All resolutions reported by the system, plus an optional custom display mode, are available from the options screen. The aspect ratio is shown as well
In widescreen the horizontal view is extended, instead of cropping off the top and bottom
The field-of-view can be specified by the user. The setting will work with both the original game and fanmissions, with existing savegames and with missions which use scripted FOV changes. FOV adjustment includes the mechanical eye and bow zoom
The HUD and menus are shown correctly, regardless of resolution and aspect ratio. In 3-monitor modes, the HUD can be constricted to the center monitor
The main menu and loading screens are shown correctly, at 4:3 aspect. The proper movies, which are letterboxed, are allowed to crop to fit the monitor (up to 16:9)
XBox360-type gamepads are working, the triggers are mapped to buttons. Support for up to 16 gamepad buttons using modifiers

As an FM loader, the emphasis is on ease of use and reliability:

Easy to set up – besides this patch there is nothing to install or configure
The contents of your game installation folder are never changed. (GarrettLoader has a habit of messing up your installation)
Works even if the game is installed in Program Files
Automatically installs custom conversations. (Used in the FM All the World's a Stage)
Fixes for crash on save and crash on exit in certain FMs, and for the mechanical eye zoom not working in many FMs
The FMSel supplied with the installer is slightly improved over the NewDark version

Additionally some optional tweaks have been incorporated:

Mirror and/or scale down the main HUD
Minor HUD and GUI tweaks: Remove loading screen tips, mechanical eye overlay, main menu version text, "Nothing" texts, loot percentage, purple fog, 'bouncing arrows', junk item flashing and the lockpicking HUD
Alternative background modes for the readable/map and pause screens
Better mouse response and removed motion blur
More realistic physics
Scale the difficulty up or down. (Can be set to zero for a 'notarget' cheat)
Disable auto-raising of the blackjack and dagger
Reduce or removInstallation:

It's recommended to disable the virus checker during installation, since executable files are going to be modified
Advanced users should consult the list of additional installation options in the readme
Steam users should read Additional notes below
Execute the installer
After installation, run the original game once and select your preferred resolution in the Options screen. (The patched game uses a private setting for the resolution)
The SneakyOptions.ini file is now available for editing

The installer will set up a start menu group with shortcuts to the readmes, the options folder, installed FMs folder etc. By default it will also create four icons on your desktop: A yellow glyph icon for the original game, a cyan version for the fan mission selector and matching icons for the readmes.

The patch has its own configuration file, SneakyOptions.ini. This is found in a folder named “Options”, located in the same location as the savegames for the original game – that is, next to (not inside) the “SaveGames” folder. All FMs will use this location as well. The original Options.ini file is also found here, and is copied in by the installer.
The options files can be found through the Sneaky Upgrade start menu group (in Local Folders). The SneakyOptions.ini file is where the tweaks etc. are configured - see the readme for a list of options. For the time being you'll have to edit it manually, there may be installer support or a configurator utility for this later... Note that all tweaks etc are disabled by default.

Fan missions:

Beginner's quick start guide to playing FMs using the FMSel loader frontend: (To play using GL, just make sure GL is installed before installing the Sneaky Upgrade.)

Create a folder for downloaded FMs in a neutral location. A subfolder of your Downloads folder will do
Go to a site which hosts Thief 3 FMs, and download some into that folder
Start FMSel through the cyan desktop icon.
A dialog will ask for the FM archive path. Select the folder from above
The FMSel window will now show the titles of the downloaded FMs. Right-click an entry and select View Info File to see the FM's readme
To start the mission, click Play FM (or Start FM if you're ready for a full play-through)
See the FMSel readme for more detailed information

Additional notes:

The patch expects an original installation, the original T3UI.ini in particular. Existing tweaks may or may not be compatible.

Steam users should disable updates for the game and refrain from doing game cache validation. If there's an immediate crash on launch in offline mode then either go online or replace the Steam specific T3.exe with the one from this archive. (Do not replace T3Main.exe.) If there's still problems try the suggestions here.

If you want to use TDS Tweaker to change frob colour, then you have to get this updated version of Myagi's patcher. Copy it into the T3Tweak folder and rename it MyagiPatcher.exe. It's recommended to use the built-in tweak instead of TDS Tweaker to remove the loot percentage.

If you have existing savegames, and they appear to have gone missing after installing the patch, then chances are that your SaveGamePath registry value is set to a location in your private user folder. (The original default path is in your Documents folder). Such paths are diverted to the public Documents folder for the unpatched game, but not anymore after patching. Either locate the new folder and move your savegames there, or edit your registry to use the old path.

There are a few known issues - nothing major, though. There is a list in the readme. Hopefully they will be fixed in a future version.

Acknowledgements:

Tester hall of fame: decayedmatter, DJ Riff, lowenz, Lolwutman, Corsair, bikerdude, Beleg Cúthalion, Child Of Karras, gnartsch, chrish, Pheonix, PsymH, Hiatus, massimilianogoi, tolsen64, Xarg, Zaratul, voodoo47, Lord Soth, scavvenjahh, scarcow and TreyM

This patch builds on previous fixes and ideas etc. from the following sources: massimilianogoi, tolsen64, Hadley, NotCarolKaye, daremo, Xiaopang, New Horizon, Pheonix, Kerrle and GOG.com...

A huge thanks goes to le Corbeau for NewDark and FMSel, and to the FMSel translators: gnartsch (German), Briareos H (French) and raven4444 (Polish)

Thanks also to clearing, Lord Soth and Esme for file mirrors.e headbobbing
No switch to third person in death
Classic speed. (Walk by default, run qualifier)
Human-readable savegame folder names