Too many icons on a slide usually just creates clutter. I often get slides from clients in which they request icons for almost everything. One of the key principles in slide design is to be concise and get rid of information that distracts from the message, so you should consider limiting your use of icons if they are becoming distracting. I'm not saying that a slide full of icons is always a bad thing, but in my opinion, it is most of the time. Here are a few guidelines for deciding when to use or not use icons:
If you think it will help your audience to remember a particular idea or category of information in your presentation by associating it with an icon, do it. For instance if you have a slide that shows the main topics that will be covered and you give each one a name and an icon, you can use these later on in the deck to trigger their memory. The icon gets associated with the topic it's being used for and helps people track the information. The more it's used, the less you have to keep the topic name next to it. However, be careful if you decide to only rely on the icon. If you do this, the icon needs to be very obvious as to what it represents.
Sometimes words alone are complicated or confusing. If you can think of a visual metaphor to help explain a complicated or nuanced topic, you may get better understanding from your audience. Photography and illustration are often used for this, but icons can be just as good and even better sometimes because they are a quick visual read.
Let's face it: Sometimes slides just look boring. One way to cure that is to add a little bit of flare. Icons are great for this as long as they fit with the overall design or branding of your presentation. You can think of them as simplified illustrations if that helps. Even just using one icon can do the trick.
Don't use excess icons when you already have a bunch of them on a slide. If you want to have them, keep it simple and only use them for top level content or to help clarify a few things. The more icons you use, the more visual information your audience has to interpret. And while you are speaking, they will most likely be switching their focus between you and your slides, so the best bet is to make things simple.
No one ever said you had to use icons all the time. I recently had a deck come my way, from a major retail outlet, that was very polished. It had zero icons. Instead it relied on great photography and sophisticated typography.
For those of you who can't get around it, let's say because your boss is requesting a lot of icons, there is a way to do it that works. Use very small icons with solid fills. Don't use large, thin line, hollow icons. These are good when they are big, but when you have a lot going on, smaller, solid icons will help maintain your white space and still be readable. How small? 1X or 2X the height of a capital letter in the text that is directly accompanying the icons.
Remember, icons need to to help your communication, not hinder it. When you try to give everything on your slides an icon it becomes less about the information you are trying to present and more about your enthusiasm for icons! Your best bet, if you want to use them, is to only use them for top level ideas, to clarify complicated ideas, or to add visual interest.
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