I disagree. I don't think there are enough technologies in Civ4 by a factor of 4 or 5; what's missing, really, is a way of balancing tech paths so that there are different paths through the tech tree which are beneficial in different ways. Different civs having different specific technologies is a simple wrong solution to this.
For ecample, imagine there's one path down which you can get to horseback riding early. Someone else can get to catapults early, but that's of limited use if the stack of catapults they drag to your city can be trivially taken out by your horsemen; and if they want defensive units good enough to protect the catapults, by the time they get that and catapults, you'll have got to heavy horsemen or knights or some other direction that helps you defend better again. Or if bribery comes back into the game you could have built lots of marketplaces and bazaars and what not and be enough richer than the invader to bribe every barbarian on the continet to come defend you, or bribe the enemy's defensive units to trash the catapults for you. Or if my notion aboiut cultural conversions were implemented, you could go in a diffferent direction again and make your culture so goshdarn awesome that the catapults sitting outside your walls would go "Hey, we really should be working for these guys" and defect to your side.
I don't want mechanisms like supply chains that are dealing with the problem of SoD by making SoDs harder; I want genuinely different strategies that allow you to overcome SoDs by ways other than confronting their strengths.