You should only put more text on the screen when you want the people to read the slide and NOT listen to you. Even the most advanced projection systems can be difficult to see at times. Strive for strong contrast between text and backgrounds . When in doubt use either black text on a light background or white text on a black background . Combinations such as red text on a blue background can often be very hard to read when projected.
Patterned backgrounds can also be very difficult.
There's also the issue of colorblindness. Choosing an appropriate text color for a varying background can be a major challenge. If you are not sure, stick with a solid background color. This prevents your presentation dropping out to PowerPoint after the last slide. Place a shortcut of your .pps file on the desktop to enable you to open your presentation with one click without searching for the file.