On the real, DOOM while he was revolutionary with the beats, flow, cadence, and birthing most artists nowadays, the fact remains that he bit off of the following:
- KMD=De La Soul
- The DOOM persona=Dr Octagon
- Various gimmicks= Kool Keith
His beat making techniques are basically what Rza was doing before it became popular. Hell, Havoc did the same thing:
- Take a drum loop
- Take a sample loop
- Match the BPMs
- ???
- Profit
What DOOM did however was the extra stutter drum technique which still blows my mind to this very day. Overall, it hurt when the guys broke up. DOOM and Grimm was like the yin and yang of styles. I remember when "The Book of Daniel" dropped and heads were like  
 
While others were swearing that Grimm was riding the 50 publicity beef route. When in reality, DOOM was the one in the wrong. Grimm brought him in and gave him game at his lowest point. All for DOOM to just treat him and the MIC like weed carriers. Grimm was heavily responsible for Operation Doomsday, but he will never get that due, and it's fine because RIP to the gawd, and Grimm is still one of my favorite rappers of all time.
Such a shame they broke up and barely reconcile until it was too late.