Buzzfeed is a prime example of this. This little-known website exploded on YouTube with short vids of Buzzfeed employees.
Some of these channels are scripted and some are completely ad-libbed. Some are hybrids, with planned setups but the people appearing in the video ad lib.
Regardless, all the top channels have high production values. Even the completely ad-libbed channels invest a lot of time preparing for the shoot. Those appearing know what’s expected and map out what they will do even if they don’t completely script everything they say.
Post production editing is critical. Wipes, slides and other special effects can be done with laptops today.
The other vital part is good sound. People must be able to hear the speakers clearly. Poor audio will kill a video.