A good act is hard to follow, and Warband is the same routine with a few new tricks added.
First thing that experienced players will notice is the menu has a new multiplayer and custom battle option. I dove into the custom battle mode first to see how it stacked against the custom battle mod for the original. sadly, it is lacking. Very few maps and you can't pick what specific units each army gets. You also can only use a handful of preset characters instead of picking a soldier from your allied faction to play as. I hope updates bring more content for this mode.
The single-player map is different from the original and resembles Europe to an extent. All the factions from before are present and a new desert faction has joined the fray. You get to choose your starting area this time and it opens with you having to fend off an attacker, then moves on to a quick opening quest to help introduce you to the game. Experienced players need not bother, but it is better executed than the previous games quests.
The biggest change is in teh diplomacy and marraige features. I haven't played enough to see how these effect the gameplay, but I never bothered with politics anyways. Those that seek conquest through war like me may not have a use for most of the changes.
A big change to combat is the weapon modes. Javelins can be switched to makeshift spears, among other changed. The most annoying change is the couched lancing (veterans will recall how by getting your horse up to speed while holding a polearm will automatically lower it and you can ram it into your enemies), now only certain polearms can be couched and you have to activate it and strike within a brief time limit. It can be aimed, but the timer and toggle bits means it will take a while to get used to.
Besides a few new weapons and armor, diplomatcy options, map, and new faction, most everything in Warband is unchanged. If you have the original, get the demo and see if the changes are worth the $30 price tag.