What are the risks involved with solo diving?

There are a number of risks associated with solo diving.  Essentially, any of the risks for regular buddy diving are also risks for solo diving, except that there is no one else to rely on for help.  Running out of air, decompression issues, hypoxia; any and all of these are double the risk when there is no one there to help.  However, most of these can be dealt with when solo divers have the proper equipment, which essentially means having redundant equipment.