Why is the Cumulonimbus cloud formation so dangerous?
A cumulonimbus cloud (CB) is dangerous because of the processes occurring in them.  At a minimum you will have an updraft and a downdraft containing rain.  The particular CB you posted in your picture is typical of severe convection.  The anvil is spreading out along the tropopause and there is significant overshooting tops over the anvil.  This indicates the updraft in this cloud is very strong and there is a lot of energy in this storm.  A storm like this likely has hail near the updraft, very strong winds both up and down and severe turbulence within the interface between and around the strong up/down drafts.  There can also be very active in-cloud lightning as well as cloud-to-ground discharges.  

Threats near the storm are turbulence, lightning, large hail and significant downdrafts (e.g. microbursts).  Threats within the cloud are severe turbulence, very strong updrafts (some as high as 100 m/s or more) and downdrafts containing torrential rain, hail and lightning.

To better illustrate the magnitude of vertical winds, here is a quick plot from a modeled storm I am working with.



In this plot the reds are updraft and blues are downdraft with the scale on the right in meters/second.  The plot covers 20 km x 20 km area at a height of 5.4 km, which is close to 18,000 ft or 500 mb.  This corresponds to the main updraft in a supercell and you can find areas where less than 5 km cover a vertical wind gradient of 50 m/s.  If you were penetrating this at any decent airspeed you would be in for a very rough ride.
 This storm only had 1800 J/kg CAPE and updrafts can be much faster than this. 



CB clouds are (typically) realized from a release of convective available potential energy (CAPE), which is simply a measure of how much warmer a parcel rising from the surface is than its surroundings as it rises.  In an environment that is favorable for convection (CAPE, vertical windshear) where there is ample moisture and a mechanism to force parcels to rise to their level of free convection you can end up with a CB cloud.  A CB is simply a cumulus cloud with rain falling out of it and whether this is a small rainshower or a severe rotating supercell is determined by the environment the storm occurs in.