Tyson never impressed me. He's never been that great a boxer. As a straight up brawler, the fact that he's about brain dead already and has tremendous power, makes him great in that sense. But as a real boxer, with the abilty to defend, and to deal with a really good defense, he sucks. In the ring with one of the true greats in their prime, Tyson's only prayer would be a lucky punch.
There have been very few really GOOD heavyweight boxers since Ali. Foreman is an anomoly (sp?), he's from the Ali era and made it into another decade or so with DESIRE. The only heavyweights that impressed me much since Ali were Lewis and Holyfield. Holmes, Tyson, and the Spinks brothers just happened to be decent when there was no great boxer around in his prime.
I figure Ali was already being affected by Parkinson's disease even before he retired, although not so you'd notice. I'm not talking about slurred speech and tremors, but more slowed reaction times and such. Combined with age and the decreasing desire, it was enough to allow guys like Holmes to come to the forefront.