Nikkor: Consistent color rendering among each generation of Nikkor lenses of the same quality. The "newest" series of Nikkor lenses produce very rich color saturation, and have an almost super-realistic way of rendering scenes, at least on the high end of things with lenses like the new 60 and 105 Micro-Nikkors, the new 70-200, and the new 24 and 35 and 85mm primes, and so on. A typical Nikkor's color rendition, which is not the same as saturation, tends toward what most Canon users might call "cool" or "neutral". Sigma has always been known for its warmer, more-yellowish color rendering, which can NOT be 100 percent eliminated through white balance adjustment. Simply put, Sigma lenses produce more-yellowish renderings of all the colors, compared against Nikkors. (And compared against Canon EF lenses as well.)
Sigma makes a lot of different lenses. They have had some quality control issues over the last few decades, with "some" of their lenses having some really bad, serious QC issues, to the point for example, that the Lensrentals.com company simply had to STOP buying certain Sigma lenses because they broke down constantly, at a high,high rate under rental lens use patterns.