Incredibly obvious as this injunction may seem, people do violate it often. It's hard to read off such backgrounds. These detract from the talk, and can make slides hard to read. For every animation you put in, ask yourself whether you really need it, and whether the slide can be explained without it. In my experience, the only good use for animation is to show a progression of events.
Animation is attractive, but often unnecessary for presenting steps of a process or algorithm.
It's completely unnecessary for backgrounds, individual bullets, comments, the first thing to appear on a slide, and so on. DON'T assume your presentation will work on another person's laptop.