Video resolution is low compared to still photographs. Even a cheap DSLR that offers HD video can offer professional results. What matters is lenses, focusing, stabilization, and sound. If you watch how pros use DSLR's for video, they use manual lenses, focus pullers, rigs, and external microphones. Plus they have the experience to make great movies.

If you are an amateur who has never had a DSLR and do not know anything about exposure and manual control, you are better off with a camera that is more automated, such as the D5100. In fact, a dedicated video camera would be even better.