I'll tell you. Size and power. However, Dempsey beat a better all around fighter than Wlad in Williard. Don't believe me, watch this film. You'll see that Williard can throw in combinations, has a very similar stance to Wlad, but more loose, and he can counter and roll with punches. If you watch closely, you'll see how the Klitschko's probably paid attention to how he would roll punches off him by rotating his shoulder and bringing his arms up. He would also punch and clinch when he missed shots, similar to Klitschko. The only difference between some of today's fighters' and the fighters of the past is explosivness; however, endurance and strength are mutually exclusive. Obviously for today's 12 rounders, explosiveness is more suited than endurance, although a balance must still be established. The point being, you give guys like Dempsey and Williard modern strength and conditioning, they would be even more dangerous over a short period of time which suits this era. I would say, both Williard and Dempsey would knock Wlad out. The bottom line is, Wlad doesn't have a chin. Watch Williard at about 2:20 when the camera zones in. You'll see him throw combinations and counterpunch. http://www.youtube.com/watch?v=dB13_HZD83Y Watch Dempsey roll punches, low, perfect to get underneath a guy like Wlad's straight punches, especially considering Wlad does not have fast lateral speed nor an uppercut. http://www.youtube.com/watch?v=mvBXligl6YI Wlad does have good footwork, especially when closing the distance, but you can get to him, and he lacks stamina to keep a constant rotation and movement. Dempsey would and it would be lights out. Williard could take Wlad's shots, but Wlad couldn't take his. Lights out twice. Wlad gets his lights turned out by both fighters. Now Vitali...that's a different story. Vitali is a top ATG. Wlad... he's a great fighter, but he's not in the upper tier of ATGs.
The majority of boxing fans who claim to study classic fighters underrate Dempsey in my opinion. I too believe that Jack Dempsey (with access to modern training and diet) could give Klitschko trouble, lots of it. Dempsey was very fast, hit extremely hard and was a very intelligent fighter (a trait he does not get enough credit for).
Video evidence doesn't lie. Suppose you saw a black and white, crap film version of the Wlad vs Povetkin fight. Would you be saying, "wow, what a great fighter Wlad is?" No, you'd be saying, "this is the worst heavyweight champion I've ever seen."
Willard was semi-retired and out of the ring for over 3 years when he got dragged into the ring to get a **** kicking from Dempsey. He also looked soft and out of shape, hardly the physical specimen that beat Jack Johnson in the heat. That said, I pick Dempsey over Wlad. But just pointing out Willard wasn't in no condition for a fight. Jess was certainly an underrated fighter at his best, though I would pick Wlad to beat him in a H2H.
But you must admit that Williard gets underrated. Sure the critics were harsh, but on film he doesn't look any worse than Wladimir. If anything, he shows more versatility and a better chin. Perhaps Wlad has more power, but Williard can throw an uppercut and if he does get hit, he won't be on his hands and knees, crawling all over the floor. I think Dempsey has the speed to get to Wlad, and the punching power to hurt him. I don't think Wlad could beat Williard either. I think Williard would be able to take his shots and defend against them as well and has the strength and height to beat Wlad at his own game, plust he has better stamina and a chin to take the shots. I believe Wlad would win the first few rounds, but Williard's stamina, chin, and punch versatility would take over and he would stop Wlad mid to late rounds. Plus Williard would actually go to Wlad's body.
Vitali would be tough. I think Vitali is a very skilled boxer, especially for his size. Vitali would be a tough fight for anybody. I don't think you can count Vitali out against anyone. Ali, Jack Johnson, and Larry Holmes are some of the more difficult stylistic matchups for him.
He is very underrated. He was actually closer to Vitali than Wlad. Good condition, very durable, hands down stance, and a very loose kind of probing jab. He did tend to get hit a lot over the top by the rather washed up Johnson here. He did do a decent job of clinching though to neutralize as much as he could, using his size to shut down Johnson's clinch punching. http://www.youtube.com/watch?v=74dnCeXI8MM
Willard didn't hit as hard as either of the Klits. He had a total of 34 fights with a 58% ko rate. The Klits are more athletic and probably stronger. Once Dempsey got hit flush by one of the K-Brothers his strategy would have changed if he didn't get put to sleep. The Klits know how to use the jab very well and have very good timing. I just don't see Dempsey getting close enough to do anything with either one of the brothers. Same goes for Louis, Marciano and Patterson.. All those guys are too small to mess with this huge guys.
Vlad would knock Dempsey out, too big, too skilled, too much power. Vlad is a totally superior fighter than Willard..............
Sorry, I have no real horse in this race (old fighters vs new fighters) but watching the videos you posted, Willard is not even close to the quality of fighter as Wlad, and its quite apparent, imo.