When it comes time to think about an animation proposal for a client, perhaps the very first question that needs to be addressed is whether the animation will be 2D or 3D. Each of these styles have unique benefits and fallbacks and are each capable of looking great, so the process of deciding comes down to figuring out which style can best meet the needs of the project.
With 2D animation, each frame can be tweaked to an animator’s liking and the variety of styles available is endless. 2D animation can be anything from an animated infographic to a frame by frame watercolor animation where each frame is created by hand. Simple, complex, informational, abstract -- 2D animation can really be whatever you want it to be. Any time an abstract idea needs to be illustrated or text needs to be brought to life, 2D animation is the go-to option because it provides a high degree of control and can make effective use of the 2-dimensional space.
Where 3D animation really comes into play for me is in dealing with something that needs to be true to the real world. For example, a project I’m currently working on involves an animated demonstration of an injection into the ankle joint. Finding this injection point can be challenging in 2 dimensions because you have to choose one angle to show each movement from. Being able to pan the camera around the ankle in three dimensions as the needle goes in allows the viewer to get more information, and creates a sense of verisimilitude that makes it easier to repeat that procedure when faced with a real, three-dimensional foot in a clinical setting. In this case, the extra work that goes into lighting and texturing the scene (not to mention added render times) is worthwhile because the third dimension of space presents real educational value.
Of course, these are only rules of thumb. It is certainly possible to create a true to life, medically accurate animation in 2D or an abstract, artistic explainer animation in 3D, and taking approaches such as these would present a unique challenge to me that I would embrace. In most cases, however, choosing between 2D and 3D for a given project boils down to assessing the project’s needs and choosing the style that is able to most efficiently address those needs based on the strengths of each medium.