As you might expect, from a nuts-and-bolts technical perspective, there are some easy wins for the PC: for a start, texture quality and filtering is generally in another league altogether compared to the Xbox 360 game. This is not exactly unexpected. Special cases aside, the standard for multi-platform development these days tends to be a case of targeting artwork for console 720p and then simply offering the option of higher resolution rendering on PC. Not so with The Witcher 2, where the developers are clearly providing superior assets aimed at looking great at 1680x1050 and beyond.
Indeed, there's so much detail in the textures here when running at 720p that there's sometimes a shimmering pixel-crawl effect - a by-product perhaps of the lack of multi-sample anti-aliasing and too much detail being crammed into what is, by PC terms, a fairly meagre level of resolution (if you have the GPU resources to spare, here's where the Ubersampling option comes into its own).
PC also scores easy wins in terms of shadow quality and the number of them being rendered dynamically. Effects work doesn't just benefit from higher-precision buffers, but also from physically higher resolution: performance-sapping alpha effects such as smoke, fog and particles are clearly running at a lower res on the Xbox 360, with intersecting geometry often showing some noticeable jaggies.
In other areas we also see a generally higher quality of presentation on the PC game: both versions employ post-process anti-aliasing, and the effect is patently cleaner on the original release - the Xbox 360 game looks to be utilising console-quality FXAA or something very similar. Texture and geometry pop-in and draw distance also show clear advantages on PC, as you would expect.