Putting as much information as possible on a slide is wrong because sometimes, that will create a messy presentation. A messy presentation can become unreadable and if your information is unreadable, it is not effective. Similarly, you do not want to use a font color that blends in with the background. A yellow font on a yellow background could clash negatively or make your information seem to disappear into the background completely. Presenting this on a screen would make it almost impossible to see anything of what you wrote. This would be ineffective, as well. Using the largest font size possible is also not a very good idea because it's going to be too big. It will fill up the entire screen and make everything messy seeming, overwhelming to look at, or unreadable. Another ineffective decision.
Now, although using a maximum of two fonts on a slide is important, this does not specify the fonts which you should NOT use. There are many unreadable fonts that create a messy an ineffective presentation, and just getting to choose any two of them isn't very specific.
The correct answer would be to use a consistent color scheme. Sometimes the most effective presentations are simple, yet well put together.
