The issue concerning color combinations is certainly one to pay attention to. Web designers will be on safe territory if they pair up highly contrasting colors with different saturation levels. The best way to test color contrast is to print off the web page as grayscale on a black and white printer. This is a standard affordance test for links.

The action issue cited above has less to do with mouseover state than it does with language. Both "Go" and "Search" are used interchangeably on websites and yet they mean very different things linguistically. The "Go" button was developed to save space -- on narrow sidebar menus the word "Search" was just too long. This is a classic example of a designer putting the look of the site, in this case, space constraints, before the meaning of the action. It's the label that causes the user confusion, not the mouseover state. The mouseover may trigger the click but a change in color does not presage what will happen once you click. The more specific the label to the action, the less the user has to "think" about what they are doing or what the result will be.