Hum, there's a mod out for Shogun 2(A compilation mod) that makes things far far better.
I haven't played multiplayer yet, though i want to. Single player is not bad, each faction has the "same" units, but the different factions have bonus(or costs) to units. Oda has the best Ashigaru in the game, while Shimazu has better katana samurai. So, even though units are still the "same", they actually applied faction bonus's to the units. Choosing a faction for it's bonus is very important.
(Beat campaign once with the shimazu as christians. Interesting way to expand: Get some high level Missionary's and then go and convert the populations and then foment revolts. Easiest way to hurt enemies and allies without being dragged into a war, though beware of enemy agents.)