Sorry man. I actually really appreciate the questions.
I'll tackle the bolded here. I'm sure
@patfanken has a thing or two to say about this as well.
Instincts and Play Recognition - are probably the biggest factors here. You're reading and playing so much more of the field at Saf so your Instincts have to be top notch. And you have to be a fast thinker in your feet. You're not playing and recognizing tells on one guy rather than watching the whole picture develop. Instead of watching one guys hips or hands your looking to see if a TE leaves the los or splits out. PA, Motion, misdirection? What are they setting us up for? "I'm the last line of defense" All these things and a lot more are going through their mind. So your instincts can't be lacking at all. And again you need to see things happen before they happen and be able to mess with a QB as much as they mess with you.
Run support and tackling are the other(s). You substitute LB for Nickels for example so they better be strong in run support. Run support is huge though. Strong, FS it doesn't matter in doesn't matter in today's game. Whether you're making plays near the line or in the open field. You can't be afraid to get dirty and stick your nose in there. Again especially when you're subbing LB out for Saf. That exchange has to make sense.
DMC is all of these things and so much more. In his prime QB's simply didn't test him. They wouldn't throw his way or at him bc he had all those things plus great ball skills to finish the play.
In terms of physical attributes? Agility is definitely up there bc you need to change direction so much. In coverage or coming off PA. Also taking on someone in the open field. Burst > long speed imo. Size certainly helps.