The exact cause of this is unknown to me, but I can certainly speculate. It appears that Eidos modified the re-released DeusEx.exe file to only run if it is called through Steam. The filesizes are the most obvious give-away: the retail copy of DeusEx.exe weighs in at 248KB, while the Steam copy of DeusEx.exe is only 62.5KB. It is anyone's guess as to what the exact differences (at a code level) are between the two; all I know is that if you try to run full-conversion mods with a vanilla install of Steam DX, you'll be met with disappointment.

I also discovered that Steam DeusEx.exe doesn't like OpenGL mode at all, even with the new renderer[www.cwdohnal.com] in place; it is as though Eidos enforced D3D functionality when they released it on Steam.

To play and use full-conversion mods with the version of Deus Ex that is sold on Steam, you will need to secure a copy of DeusEx.exe from a non-Steam copy of the game. The best way to do so is to:

• Download a copy of the Deus Ex 1.112 patch[www.google.com]
  
• Extract the contents of the patch to your desktop
  
• Grab the DeusEx.exe file from inside the System folder

Afterwards, replace the Steam DeusEx.exe with this retail DeusEx.exe. All custom Deus Ex mods that you install from this point forward should Just Work™.

If you installed a full-conversion mod prior to performing the steps above, you'll need to replace the exe that shares the mod's name with a copy of retail DeusEx.exe renamed to <mod name>.exe. For example, if you previously installed The Nameless Mod, you will need to replace the existing 'TNM.exe' with a retail 'DeusEx.exe' renamed to 'TNM.exe'. The reason for this is that most full-conversion mod installers make a copy of DeusEx.exe that's found in the base game's ./System/ folder. Custom exe's will fail to load because they are copies of the crippled Steam DeusEx.exe.

I mentioned earlier that OpenGL doesn't work with the Steam install of DX. If you replace Steam DeusEx.exe with retail DeusEx.exe, you'll gain back that functionality. This is important to remember if you install a mod like New Vision and have issues with it loading.

A friend of mine asked, "isn't TNM.exe just deusex.exe with some parameters strapped in? If so, could you just add the parameters through Steam?"

Technically, yes. Full-conversion mods that copy and rename DeusEx.exe can be launched through Steam. To do so, you will need to add additional launch parameters to their entry in the Games list when you add the mod to Steam:

• Add the mod's executable to Steam as a Non-Steam Game
  
• Right-click the mod from the Games list and select Properties
  
• Click the Set Launch Options button
  
• Copy the "Target" string from the desktop shortcut into the shortcut's Launch Options box

You should now be able to launch the game from Steam without issue.

It is pretty simple to get full-conversion mods to work with a Steam-based copy of Deus Ex - all of the assets are the same between this copy and a physical copy sold over ten years ago(!), except for the DeusEx.exe file that has been modified to incorporate Steam DRM. The entirety of this document can be summed up in three steps

• Grab a copy of DeusEx.exe from the freely-available 1.112 retail patch
  
• Replace the Steam copy of DeusEx.exe with this retail copy
  
• Install your full-conversion mod

Just follow these steps and you'll be on your way to enjoying some great mods!

P.S. Mods that affect vanilla Deus Ex gameplay should work fine with an unmodified Steam installation. You still might want to replace your Steam DeusEx.exe if only to get back OpenGL rendering - there is a far superior custom renderer that blows the default Direct3D renderer out of the water, and the only way to enjoy it is with a retail DeusEx.exe.