I assume you've seen the Demoman's new equipment. Most unlockables for TF2 have been things that change the class's role in a mild way: bow and arrow makes for a mobile sniper, but he still shoots at guys from a distance, Backburner encourages attacking from behind, but you still run at people with the hopes of them burning to death. The Eyelander and Chargin' Targe are a different animal all-together.
The sticky-launcher is an odd item: it's good at anything but the longest and closest ranges. It can get instant-kills with a couple seconds of preparation. If a Demoman is hiding at the end of a narrow tunnel, there are few ways to approach him and not be killed without even seeing him. Additionally, the stickies themselves do a lot of damage, and can be detonated in mid-air, making aiming a lot less difficult, (especially since they have the largest blast radius of any TF2 explosive).
Let's compare that to the Shield. By itself, the shield makes you the toughest pill-shooting guy on the planet, with essentially double health against explosives and fire. And you'd be capable of an occasionally devastating charge that goes faster than a scout. If nothing else, he could use it to retreat without possibility of being captured. (Unless it requires some sort of cartoony "wind-up" period, where he takes a step back before he charges. That would explain the difference between medium and long-distance charges in the explanation: different wind-up times.)
With the Sword and Shield together, the Demoman becomes something more like a pyro that gets bonuses when he gets kills. Charging people that have become isolated looks like it will be a devastating strategy; you can get a guaranteed critical hit with the shield, and get the health and speed bonus to hit the next one even faster and harder. The only problem I see is that it'll be almost worthless against a large group, unless you get them from behind.
The charge in particular raises so many mechanical questions: will you be seeing yourself from third person, like the scout's "bonk" drink? Will you be able to "pull out" of a charge? Will there be that "wind-up" period before you can execute the charge? (If there is a wind-up period, that would actually remind me of the stickies; you lurk around the corner, then trigger the attack as the enemies approach, then you get an instant kill.)
PS. It looks like the Sword/Pickaxe theory from the monkey comic was legit, we can expect the Pickaxe for the soldier today. I don't really care for it thematically. Soldiers did use entrenching tools like shovels, but it's not like all they did was dig holes. I would have expected something more like a trench knife, much more soldier-ey.
PPS. A while ago, some people tried to do a "medieval" game on CP_Castle, natch. You could only use Huntsman, Jarate, and Kukri as Sniper, and you could use the Flamethrower, (but only to ignite arrows,) and the axe as a Pyro. All the other classes didn't have appropriately medieval weapons. But with the Demoman's new stuff, it's like an SCA meeting in TF2 Land.