We have two big online features, which I can't really say any more about. It's just not there yet and we're not comfortable talking about it. But both of these features are something that you have never seen before and will be a big part of the game. I don't think unique units would unbalance the game too much. Better units usually cost more shields, i.e. build 2 crappy MIG fighters for the same price as one F16 fighters.
Power lies in numbers.
Also, having unique units in Civ3 could make it much easier for scenario designers to give specific units to a particular civiliztaion. I also don't think unique units idea is essential, but it could make the game more interesting if it's properly balanced. I think this could work really well with the new resource system. Everyone will need a specific resource to make their special unit. For instance, Rome needs iron to build the Roman Legion.