A lot of them looked fine before the piano lacquer abomination. Aluminum is classic, but other variants where OK, too (why not carbon fiber?). I guess there is also experimentation.

Also go for simplicity, I suppose. No fake parts designed to look "high end" in a phony way (kind of like "spoilers" on cars).

To me the utmost example of bad taste is the leather coating ASUS is going for in their latest creations. But then I don't like leather in general.

Just try to remove as many superfluous parts and elements as possible I guess. Just as Apple does - battery covers are superfluous. Mouse buttons are superfluous. Multimedia buttons are superfluous. Shiny areas anywhere but on the display are superfluous. And so on...

Important: large borders around the screen are superfluous and look cheap.

I can't think of a situation where multiple colors would look good (ie notebook cover red, bottom black), but I am not a designer.