The shape's influence
Consistency is important when deploying graphic elements in a presentation. What kind of message do you want to send? What do you think your audience expects from you, and are you willing to do the unexpected? Is it important to invite critical thought and discussion, or are you trying for a specific outcome?
Shape characteristics have a part to play in setting a tone and conveying meaning.
People see curves as friendly and corners authoritative. Thus, circles are good for expressing items without a particular order and rectangles lend structure to your message. While rectangles are easy to align and make uniform, circles tend to overlap and appear to be in motion even when they’re not.
If you mean business and you want your audience to know that, opt for rectangles. If your objective is to elicit a response, curves can put people at ease and encourage talking. Rectangles with rounded corners provide a happy medium, but this isn’t the only alternative (it’s a troublesome one as well — achieving consistency with rounded corners is an advanced skill).
Triangles are generally impractical as text-boxes, but they tile very nicely: A collection of triangles can be flipped, rotated, and re-positioned to form a larger shape, of which each triangle appears to be a component.
Even better is the hexagon. A happy medium between circles and squares, the straight edges give hexagons structure and authority, while the obtuse angles make the corners softer and less standoffish. Personally, I find the hexagon transfixing and am fascinated by patterns a designer can make with them. Good use of hexagons can electrify a presentation.
Here’s a little more …
Arrows with triangular arrowheads (—►) are definite, while open arrows (—>) indicate flexibility
Arrows with round arrowheads rather than points are even less definite, practically begging to be re-routed
Trapezoids look heavy and convey seriousness
Outlines lend weight, shapes lacking outlines seem vague, especially if their color doesn’t contrast with the background
Star shapes stand out but can make people uncomfortable.