It depends on what your objectives are for this assignment. Are the students presenting these or simply making a PowerPoint for you to look at? Is the content more important to you or the fancy bells and whistles (like slide color, transition, fonts, pictures, etc)?
When I assign a PowerPoint presentation, I make the kids present it to the class. Here are a few things I look for:
1. Answering the question(s) or topic thoroughly.
2. Ease of readability (can I read it without get distracted? Is the font big enough, clear enough, etc?)
3. Presentation skills (rate, pitch, volume, etc).
4. I can't stand kids who just read from the PowerPoint, so I take off points if they don't use it as a source of speaking points.
5. You can make it a requirement that they include one picture or video or something.
6. If you're focusing on the use of PowerPoint and their skills with the program, you may want to evaluate their use of transitions, different types of slides, background, etc.