Played the original? Good. Haven't played the original? Even better.
For those who haven't played it, here goes:
Mount & Blade: Warband is an Action RPG based on medieval times. (Though not the real life medieval time)
You basically use swords, bows, axes, lances, crossbows, etc.
This is indeed an expansion. And for those who don't know, 90% of the expansions are mods. (Company of Heroes: Tales of Valor, Halo: ODST anyone?)
Now, they added a lot of stuff. They added a nice amount of armors and weapons. (No horses sadly) They also added a new faction, the Sarranids. Added a new marriage system in which a guy must fight for the heart of his beloved and a girl has to pretty much search for the man that thinks she is feminine enough.
The best thing, like almost everyone says, is the new multiplayer in which up to 64 players (Not 32, Gamespot) can fight each other. So it's 32 vs. 32. I personally haven't played the multiplayer and I'm probably not going to since I practically hate multiplayer, but for those who do, this is a great add-on to the game.
Also, let's not forget that the player can now have it's own faction and vassals. Now that's another awesome thing.
They have modified several things. They changed the faction colors for reasons I don't know, they also changed the map to apparently fit the new faction in there and most importantly changed the graphics... for the better.
Now, also seeing the negative stuff... They changed the horseback combat for the worst. Now you have to literally be next to your foe to be able to hit them with your sword... like seriously... you need to be NEXT to them, not even a centimeter or even half a centimeter away from them.
The morale system was also damaged in my opinion. Your morale drops like it always did, but now it's hellishly hard to bring it up. You need to keep winning battles in which you're outnumbered and the siege battles don't give any morale. As of date of this review, that bug hasn't been fixed.
Another negative thing is that, as the game explains, playing as a female character is like "hard mode" now. The men in Calradia seem to be very sexist. So sorry girls, it's hard mode or playing as a guy.
For a last negative view... is that if you already have the original game... $30 is just a lot of money for an expansion... that's practically what Medieval 2 Total War: Kingdoms costed when it just came out! (And that, my friends, was a pack with 4 expansions) But for those who don't own it yet... by all means go ahead and buy it!
Overall, they did a great job with this game. It's definitely a good idea to play this game. Don't let yourself get fooled by the first impression or the pessimist people who say bad stuff about this game. Try it... and you'll love it so much you might just become addicted to it.