With the transition of 3D animation by Disney, starting with Bolt and then Tangled, Wreck-it-Ralph, and soon to come Frozen, sometimes I hear a comment that somewhat amuses me. People would say that they wish Disney would go back to the traditional 2D animation. Disney did to try and go back for a bit with Princess and the Frog but the movie didn't do quite as well in the box office as they would have liked. While, yes, there is something beautiful in the old 2D animated feature films produce by Disney, 2D animation is extremely difficult. I would probably say it ranks, in my mind, as one of the hardest thing to do as 2D animators need to have insanely strong drawing skills, being able to draw anything in any pose and in any perspective along with having to keep the drawing sold and the form consistent throughout all the frames. Along with the technical difficulties there exists the fact that 2D animation is less cost effective than 3D animation.
The very first issue that prominently shows up in 2D animation is the differentiation between the actual animation and the background plates. Such as in the early Disney movies, and even in current Japanese anime today. The backgrounds are extremely detailed and beautifully painted in watercolor while the animation is done on cels. The issue with cels is , through the ink and paint department the lines and color are done as flat solids, a clearly different quality than the backgrounds. With the evolution of technology and the transition to digital ink and paint, the process has evolved so that the rendering has been able to become more detailed, with shadows and lights, but it still lacks the high refined detail of texturing that can be seen in 3D animation with things such as fabric weave. It would be ridiculous to expect precise fabric weave, complex patterns, or wood grain to be kept consistent and all kept track of through both animation and ink and paint. Such was a problem in 101 Dalmatians where there were a 101 dogs each with their own individual spots. To resolve the problem, Ub Iwerks created a Xerox process where the drawings from the animators are directly scanned in and finaled, cutting out the process of the ink and paint department having to re-track down each of the spots.
Disney has actually been experimenting with 3D pretty early on. Another factor that 2D animation could not easily do were dynamic camera movements through environments. The backgrounds tend to be one large matte painting that can pan either up/down and right and left, but full on rotations and fly-throughs would be extremely difficult. While John Lasseter was at Disney, a short 3D animation test was done to show the ability for dynamic backgrounds but unfortunately the company wasn't quite ready to embrace the new technology as it was encroaching into the field of the established 2D animators. 3D was further developed later one though. Surprisingly it makes an appearance in Beauty and the Beast. In the Be Our Guest song sequence, the dancing plates and utensils in the back are actually done in 3D. Then in Aladdin, there was a full character in 3D. As it would have been difficult to consistently keep the pattern of the character as it moves, changes shapes, and flies about, Carpet was created in 3D. Next up is the integration of 3D in a key emotional sequence and that is the wildebeest stampede in Lion King.
Now Disney Animation has moved fully into 3D animated films and unfortunately has seen the departure of their 2D animators. Paperman was an amazing collaboration of 2D animation and 3D animation and I'm still hoping to see if Disney will one day bring back 2D animators and do a film in that style.