Character models and structures all have quite low polygon-counts, and most walls have pretty low-rez textures. At its worst – which is basically any indoor location, most towns, and the overworld – it looks like it was released a decade ago.
At its best, Warband looks like the missing link between Morrowind and Oblivion. In this sense it is very similar to the Elder Scrolls games, except not even remotely technically groundbreaking. The game definitely nudges you in certain directions, but following those suggestions or completely forging your own path is ultimately up to you. In Warband, like the original, you take the role of an individual on a fictional landmass known as Calradia, and are basically free to do whatever you want.
Being a stand-alone, you don't need the original game to play. Warband is the stand-alone expansion to the original Mount and Blade, released towards the end of 2008.