I mentioned it briefly in my rant, but 90s design was definitely different than today's.

Today, design for most objects is thin, shiny, and rounded, and typically matte white or blacker-than-black.
In the 90s, it seemed that there were a few different design trends, starting in the early 90s with the wood texture from the 80s, going on to off-white and square by the mid-90s, and ending with dark grey and glass in the late 90s. "Grunge" fonts were popular in the mid-90s, and it seemed that simple black text on a plain white background was the design for many ads.

http://en.wikipedia.org/wiki/Snow_White_design_language is probably a pretty good example of 90s design, even though it officially ended in 1990. It seemed that other companies used that design pattern for many years after that.

What would you say the defining traits of 90s design were?

Some linger today, such as the swoosh in many logos, such as AAA and Nike. Also, this was the decade in which Humanist San-serif fonts exploded into popularity, such as FF Meta, FF Officina, and Trebuchet MS, starting to replace some of the older styles.
Also, you still see simple black/grey text on a white background today with many Apple products. The vertical line bar, influenced by the Internet's web design also took off, and continues to this day. Really, you'll see that going through most ads in newspapers/magazines in the '90s is that it was really a bridge from the '80s to the 21st Century, with some grunge fonts along the way.