A poorly designed slide is one that is difficult to read. The readability of your slide during your presentation is affected by several factors, including room lighting and the room size. Make it easy for your audience to follow your presentation by choosing font colors, sizes and styles so that they have no problem reading your slides, no matter where they are seated in the room.
Choose font colors that contrast sharply with the background. Be sure to consider the room characteristics where you will be presenting, when you choose your colors. Dark text on light backgrounds show up better in rooms with some light, while light text on a very dark background is easiest to read in a very dark room.
Font styles also come into play. Save the script type fonts for other things as they are hard to read in general and are almost impossible to read on a large screen. You can't go wrong with standard fonts like Arial, Verdana or Comic Sans.
In OpenOffice Impress, the default font sizes are 44 point text for titles and 32 point text for subtitles and bullet points. These sizes were selected for a reason -- they are the minimum font sizes you should be using for standard fonts (some fonts may require an even higher point size to achieve the same effect). However, be sure to be aware of the room size for your presentation. If the room is very large, you should consider increasing the font size. Otherwise, the audience seated at the back of the room will have difficulty reading the text.