Ugh. Unique tech trees. Not such a great design decision, probably.
Not much added to the game (replayability? hardly.. more techs to learn?), but created serious issues to resolve and design around. There's a reason that it "had never been done before" - it's really friggin risky. I guess they pulled it off, but barely.
Hell I still don't know how tech trading works now, so it's usually disabled. There's something to be said about not making things TOO different. It makes game balancing much harder. I don't even want to begin figuring out which races are over/under powered now.