have posted an interesting article about why Doom 3 is possibly the worst idea in the history of gaming. They go through all the possible reasons why they think id Software are mad to even consider it. They say, quite rightly, that "at the moment the announcement occurred, thousands of fans worldwide promptly began setting their expectations too high".
They follow with "everything that made Doom the magical catalyst for the first person shooter genre no longer applies", pointing out that 90% of the people who made Doom what it was are no longer working for id. They finish with "Doom 3 might not even suck (and regardless of whether or not it does, it'll sell a decent amount of copies), but can one expect it to live up to the original? I doubt it". It provokes a lot of thought and is well worth a read.