Please compare M & B original and M & B:warband. I can't do on-line b/c of slow DSL. I love this game b/c it goes on forever. I have played as 6 characters for different factions. I have limited my self to about a 1000 days (player's time).
Most obvious difference is how the game area is expanded to the north to accommodate the new Sarranid faction (pre-muslim persia).
There's a bunch of faction management improvements, largely to do with starting your own faction, giving fiefs to lords on your side or your companions.
Faction troops are reworked, there's a whole bunch of new and changed items.
The graphics engine is somewhat improved in how things look, though the game also runs more sluggishly for some players. Troops have varied equipment, so eg. some knights (all of same type) fight with lance, some with 2-handed sword and some with sword and shield.
And the combat controls are slightly changed, which frustrates some players a whole lot, while not bothering some at all.