100 Block in Morrowind reduced damage by 100% because all blocks, at any skill level, reduced damage by 100%. The skill level only determined how often you blocked, because you couldn't do it manually.
In Oblivion you could block manually, and so they had to change it up. In Oblivion Block determined how much damage was reduced. However, it was never 100% damage reduction.
In Skyrim, Block will work pretty much like Oblivion, where skill determines damage reduced but never reaches 100%. With some perks maybe you can get it up to 100%, but even if you can that doesn't mean 100 Block skill without perks is any weaker than in Oblivion. The perks are optional and they'll make you a better fighter, but you can spend them elsewhere and still be a formidable fighter. 100 skill and no perks will very likely be more powerful than, say, 50 and perks.
Yet in Oblivion just having a 100 you could
* An Expert blocking with shield has a 25% chance to do a knockback counterattack, staggering the enemy if the shield blow connects
* A Master blocking with a shield has an additional 5% chance of disarming the enemy with a successful knockback counterattack
Now those both seem to be perks
With 75% block you could block arrows for a 50% damage reduction.
Another change into a perk
With a shield you block 75% of damage at 75 to 100
I think to get to the level in Skyrim your going to have to take some levels of shield wall
All in all 100 in skills now means far less then it did in Skyrim if the block skill is representative of the rest of the skills and perks