I like genuine hardwood, no veneer. Walnut, cherry, or even oak is great. Wood has a warm look that glass and steel never have. I want it to be reasonably priced. I don't want it too shiny, as I do not want light reflecting off of it as a distraction in a darkened room. I like things to be sturdy and well made.
I currently use something that is made of black painted steel and solid pine shelves, as price is a big factor for me. It is not ideal, but I have no plans on replacing it anytime soon, as it isn't bad, and it does the job quite well.
I also like modular designs that allow one to choose its characteristics, like how tall it is, etc. This is another reason why I have what I have.