Watch Dogs appeared to suffer a visual downgrade from its initial showing to the final release, but some clever PC tinkerers seem to have found a way around that. By exploring the game files and changing the values of certain values, a modder says he was able to bring back some of the effects missing from the final product.
Modder "TheWorse" on Guru3D (via PC Gamer) has created an "E3 Bloom" effect, bringing back many of the visual flourishes from its first demo. Specifically, it mentions bloom effects, headlight shadows, NPC density, and better rain effects. It's worth noting that this modifies game values, but doesn't actually add anything that isn't existing in the game code.
The community is split on whether this can properly even be called a "mod." Whatever it might be, though, the effect is impressive, and implies that Ubisoft hobbled certain effects in the PC version. Fans have been speculating on why Ubi might have done that, and explanations range from stability to parity with its console brethren. One piece of the code even purportedly indicates that the settings were turned on for the E3 2012 demo. Shacknews has contacted Ubisoft, and will update as more information becomes available.