Skip to the bottom of the review for a summary of things I thought were good and bad about Warband. The update I posted before has been moved to the DRM section.
Mount and Blade: Warband is a standalone expansion to Mount and Blade. It adds several new features, such as a multiplayer mode and some changes to the singleplayer mode. As such, I think it would be a good idea for you to wait until the price drops if you bought the first Mount and Blade, and you aren't interested in Warband's multiplayer. If you didn't get the first M&B, I recommend that you buy Warband.
Graphics: Warband's graphics are like M&B's graphics. However, there are added options (such as antialiasing) that improves Warband's appearance. Character animations have been redone. I like the new animations, as they seem smoother. Attacks while on cavalry no longer go right through your horse's head. However, the attack animations don't look as forceful as the animations in the first M&B.
Certain old meshes, like the horrible looking bandit vest, are still present in the game.
General Additions/Changes: The gameplay has been improved with additional features to spice things up. For example, players can now kick enemies, making their foes open to an attack. Throwing weapons can be switched to melee weapons with the press of a button. Enemies are more difficult to hit because the hitboxes were changed to be more accurate. Lancing now has the option of being like it was in the first M&B, or it can be made more difficult to use. The new faction, the Sarrinid Sultanate, have a very unique and interesting feel to them. There is a much larger variety of banners to choose as well.
Singleplayer: The singleplayer mode has been improved quite a bit. Instead of simply starting at a random area, like in the first game, you can choose one of five castles (one for each faction); there is also a starting quest. Factions no longer need the player's help in starting wars or making peace; they will do all of those things on their own.
There is a wider variety of areas. Villages and castles now have a more unique feel for each faction.
The player can now become a king or queen, and may marry a lord or a lord's daughter.
There are more characters and scenarios in the custom battle mode. I do find it strange that you can't just put your character in the custom battle mode, though.
Multiplayer: I find Warband's multiplayer to be enjoyable. Like in the singleplayer mode, you can give your character a unique appearance. There are a few game modes to choose from, like deathmatch, team deathmatch, capture the flag, and siege.
Server hosts can set the number bots that will spawn in the game. When you join a game, you choose between two out of the five factions, which are either random or chosen by voting players. Once you choose a side, you pick a class, which is different depending on which faction you chose. Then you choose your equipment (which is based on class) with money that you start out with and continue to earn as you gain points.
While the multiplayer is fun, Taleworlds has some work ahead of them. Weapon ranges are awkward: like in the first game, your spear will occasionally go right though a player and not do any damage. The factions are somewhat imbalanced, and some classes have much more equipment to choose from than others.
I consider the current multiplayer to be similar to that of the Stalker series. The multiplayer for both games lack some of the great parts from the singleplayer experience. In Warband's case, there are no stats and there is a lesser variety of equipment to choose from.
I really wish that Taleworlds worked on a cooperative mode instead of a competitive mode; Mount and Blade seems like it would have been more suited to keeping the depth of the singleplayer.
DRM: The DRM for Warband is just like the DRM in the first game, which means that you have to register you serial key online. However, this DRM is not as imposing as other types of DRM, like certain types of Securom or Steam. Instead of being completely unable to play the game if you don't register, you can get to level 8 and play for 30 game-days in the singleplayer. Once you register, you don't have to stay connected to the internet to play.
UPDATE: I made a mistake on the DRM section of the review. The boxed version installs Steam. However, the game itself doesn't actually need Steam to run, so once you register your copy with Steam/Warband's DRM, you can play without Steam. If your key that you got from the box doesn't work, there is another method of playing without Steam that is below. I apologize for my mistake.
Go to the Taleworlds forums if you want to remove Steam. The information required is at the bottom of the first post in the thread called 'Forced to Run Steam - Not Amused'. Unfortunately, I can't post a direct link.
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The Good: Multiplayer mode, improvements to various aspects of the first game (single player, graphics, gameplay).
The Neutral: The new animations might not be for everyone.
The Bad: Game might not be worth the price if you're buying it for the single player. Certain old meshes are still present. The multiplayer mode needs to be balanced. Some problems still persist from the first game, such as weapon ranges. The lack of a cooperative mode is somewhat disappointing.
Overall, I liked Warband. It's not drop dead amazing, but it is a good expansion to a good game. It seems worth the price to me, though.