He was a lifelong chain smoker, so I imagine he had heart disease. IIRC he talked about wanting a Houdini pulled with him after he died, so that will be interesting if it happens. RIP
He was one of the greatest interviewers ever - he could pull an interviewee in any direction he wanted, and when he tried he could unravel weaved bullshit with a single line. Granted, most of the time he just led interviews to appeal to the largest audience (*coughEdDamescough*), but he did throw a bunch of frauds off his show when their bullshit became too idiotic. Rather unlike the current main host of Coast to Coast, Snorey Noory.
There was the whole thing with him being responsible for the Heaven's Gate incident - he announced that a UFO was seen in Hale Bopp's tail, which led to the cult offing themselves so their souls could join the "mothership".