Everything I have read to date indicates that modding DOOM 3 will take some serious programming. I could be wrong. We haven't heard jack or shit about DOOM3 modability for over a year. Last year the word was that it will be MUCH harder than modding Quake 3 was. That might have changed, who knows?
I do know, from reading official statements from Valve, that Source was built from the ground up to cater to the modders out there. Valve knows where the majority of their money has been coming from the past several years, and they built Source to continue that legacy. If you haven't read about some of the killer things they have done to make Source mod/mapper friendly, then you need to get caught up. The fact that you applying textures to objects also makes the object take on the appropirate physical attributes is a big one. Want a wooden door that will interact in the game world like a wooden door? Simple, apply a wood texture. That's huge by itself....
It's just speculation on my part, but I would be willing to bet Source is WAY easier to write good solid mods for.
Want an example of an engine that isn't mod friendly? Unreal. How many working mods are there for Unreal Tournament 2003 after 6 months? Very, very few, which is why they are having the "Make Something Unreal" contest.
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He cut the possum's face off then cut around the eye socket. In the center of the belt buckle, where the possum's eye would be, he has placed a small piece of wood from his old '52 Ford's home made railroad tie bumper. Damn, he misses that truck.