A change of engine is not something I take very lightly. The decision was made over many weeks of toying with both Doom 3 and with Half Life 2, and testing the limitations of each engine. In the end, though, my decision was to move to Half Life 2, and for a few very important reasons.
Doom 3, while it lent itself extremely well to the dark and scary atmosphere we needed, did not work well for us in any other way. Not only was the lighting restrictive in outdoor environments, but the textures and assets available to us with the engine were next to none. Making custom textures, custom models, custom scripts, custom... everything was not what I had intented for the mod. I wanted to take the gameplay of Doom 3, and put it into a different setting, with different weapons, and an actual story. That proved more frustrating than I had first imagined it would be, and the further we progressed with our conceptual work the more we realized how restricted we were when using the Doom 3 engine.
Then HL2 was released and it's amazing use of physics really won me over. The graphics are not better than Doom 3, but with a proper artistic team and some nice level geomotry it will be possible to make, in my opinion, outdoor levels that look much better than Doom 3 would. So, that is the main reason we switched to Source... now let me fill you in with some new features that will be possible using the Source engine.
LOTS of enemies. Toying with the spawning commands in the console allows you to spawn as many enemies as you want, having 20 - 30 enemies on screen doesn't slow my computer down very much, and it will be entirely possible with some new models and animations, to get a whole horde of zombies attacking you.
Barricades. Yes, we had them in the Doom 3 engine, but they were static and couldn't be moved. Here is a scenario you will be presented with in A Terrible Secret. You enter a house with some chairs and a table. A horde of zombies appraoches. You can either throw all the junk in the room against the door and block the zombies for a few minutes, or you can pick up the chairs and beat them over the head of the zombies. Of course, after using chairs to hit 2 or 3 zombies over the head it will shatter, and you lose the option to then throw it in front of the door. The use of physics in Source is one of the, if not the, main reason that we changed engines, and I think this scenario gives you a good idea as to why.
The ability to carry only two weapons at a time (ala Halo 2). We feel this will add a bit more suspense to the game, and make the player think twice before pumping that zombie full of lead. Ammo will be limited and you will have to use things in your environment to defeat the zombies. I mean come on, if we gave you a machine gun with 100 bullets, the zombies would have no chance! ;)
Finally, with the amazing voice acting and model animations from the HL2 single-player campaign we got inspired to do some of that ourselves. To tell you the exact details would spoil some of the plot, so I will just tell you that you will have encounters with other humans, and it should be interesting.