Native gameplay, within the Calradia setting, with a strong emphasis for improving convenience, gameplay and adding new options (such as quests) to the overall M&B: Warband experience. Any version of M&B warband from 1.153 to 1.158 will work, but the mod requires the use of the Warband Script Enhancer v3.1.5 which is included in the download.
Silverstag is a native expansion/overhaul that delivers new key features, polish, graphics, and content, all while staying to true to the native feel of Warband.
Silverstag adds many new features but some of the most impressive ones are enhanced tournaments, sprinting (for player and AI), kingdom/fief management, and most importantly abilities and troop recruitment.
Even little things like combat hampering (soldiers fight worse when injured) dismounting damage, and spear bracing add a whole new layer of depth and attention to detail.
Player abilities, are essentially perks that add passive bonuses to players and troops. These perks can be chosen every 5 levels, and add another RPG element to Warband.
The recruitment system is a radical change from native and most other mods and has huge potential; it is slightly incomplete as of now, but the direction that the team is going in is very promising.
END 2014 UPDATE: Nearly every faction has an overhauled troop tree that takes advantage of the new recruitment system now.
There really are too many features to explain here. If you have not tried Silverstag, you are truly missing out. There aren't many mods our there that add this much depth and gameplay refinement.
I highly recommend this mod. Be careful though-- once you try it, you might never want to go back to native or any other mod out there.