Novogrod pre VI teaches you how to make do with next to no good units. Your armies will consist of boyars (OK but overrated IMHO) halbardiers and arbs. In fact its pretty dull after a bit unless you start in High and treat the campaign as a beat-the-horde game, which is fun.

Byzzies teach you the importance of good moral since Byz inf, which I like as a unit and rely on, have poor morale. So you need to manouver them on the battlefield in ways that keep them happy. And kataphractoi and VG teach you that sometimes a completely mindless head on charge can be very entertaining, as if you needed to be taught that...