It is too early to get details about magnitude and special characteristics of the spate of tornadoes the U.S. is experiencing at the present time. But when more is know, I will discuss why they were so deadly. Determining the magnitude of tornadoes is possible only after the fact, when scientists can actually examine the impacted area.
In the meantime, for some basic science info, see this article on the basics of the hazard and why forecasting them is so difficult.
Updates: Here is one early account of the first tornadoes of this season in the U.S. See Eerie Calm Preceded Violent Swarm of Super Cells. [Thanks to Pierre Picard for this link.]
The tornadoes are still active for the third day over the same large region of the U.S. See this article in the Christian Science Monitor. Just one more unusual aspect of this outbreak of storms.