@...Naked:

Using a graphics engine is different than a mod. A graphics engine is what enables you to play a game in full 3d rasterized or ray traced glory. A game mod is when you either add on weapons,items, vehicles, missions or quests or in the case of Elder Scrolls new lands. You don’t destroy the core game and just use it’s graphics engine from the ground up and call it a mod. You actually modify the existing game. When you use a graphics engine from another game WITHOUT using the core gameplay structure you are creating a new game. For example Fallout3 could technically be considered an oblivion mod because you have taken all the game mechanics from oblivion and rebadged them and presented them in a new way. Where as with Gears Of War or BioShock you are using the UT3 engine but almost every single game mechanic is completely different than UT3 hence the game is not a mod but a brand new game that is using another games graphics engine. From there they’ll add on other physics engines and AI scripts and all those other juicy goodies.

Left 4 dead doesn’t have anything out of common with halflife. All they did was take the half life SDK and create new missions and characters and left the core game mechanics present in half-life in. They didn’t take the source engine and create a whole new style of game like they did with Portal where the only thing it had in common with half-life was the black mesa and aperture science background. It is a completely different game. So in a very huge way Left 4 Dead is a half-life mod. The only reason it might not be one technically is because it is not an OEM or player made add-on that you use in conjunction with the original half-life 2 game. It is essentially the same game play only with a new story line and UI.