A prequel needs to be two things: 
 1) It needs to have well developed and likable characters (not blatantly filing off the serial numbers off old characters and insisting that they are 100% totally new), along with great story that can stand on its own two feet. If it is well executed and crafted, it will cause people to be interested in the other books in the series. If not, it just succumbs to the plague of the prequel. 
 2) It should strike a balance between giving new background information to the reader and making sure that newcomers to the series should have a general understanding of what is going on/what makes the universe ticks. Cassandra Clare has said on her website that one doesn't need to read the Mortal Instruments before reading The Infernal Devices and vice versa. But as I was reading, there was times where I felt lost and had to consult the Shadowhunters wikia page for assistance. {For the record, I have only seen the crappy movie and read a sporking of City of Bones} 
 Full review coming soon.