Don't mix 2D and 3D elements on slides/in presentations
It's important to keep the look on a slide – and within a presentation – uniform and consistent. One way to do this is through the use of 2D and 3D graphic elements. This is the same principle as using photos and clipart on the same slide: it can be done, but it's tricky.
There are basically two ways to bring 3D onto a page: 1) through the use of drop shadows – shadows tend to lift objects off the page, creating the third dimension; and 2) through the use of objects drawn in 3D. Either way, you're adding 3D "volume" to the slide. Therefore, if you use drop shadows on one object on the slide, use drop shadows on all objects on the slide. Free-floating text is usually exempt unless it's boxed. Similarly, if you use 3D objects on part of the page, use 3D objects on all of the slide. Again, free-floating text is exempt.
If you are using, for example, a 3D object (like a box) as a special placeholder for lead-in or take-away text, then the 2D/3D consistency best practice is replaced by using the boxes consistently from slide to slide.
How you blend your objects on the page has a direct effect on the level of professionalism conveyed by the slide.