He is the pioneer of Thrash Metal and along with Cliff Burton, the only reason why Metallica had good music (he wrote their best songs on the first two albums).
As a band leader, he has the reputation of being a dictator but incidentally, this is also one of the main reasons why Megadeth has some of the best music in this world. Album after album is consistent unlike their rival Metallica, who again, incidentally, began to suck after Dave Mustaine's departure and Cliff Burton's demise (seriously, haven't you noticed that?!).
Dave Mustaine is a musical genius and is very charismatic.