The Defiant seems to be the first Starfleet vessel who's armor is actually able to stand up to contemporary weapons. All other starships seem to crumple in seconds like tissue paper when not protected by shields.

I think that ablative armor used by early Starfleet vessels gave way to the more versatile energy based shield systems. When building ships, particularly large ones it takes a significant investment in mass and volume to devote to mass-based armor that goes up by the inverse square law or something like that as the ship surface area gets bigger. For a smaller ship however it becomes more practical to use it, and having a combination of both armor and powerful energy shields was possible for the Defiant with it's insane power output for it's size. So with the Defiant, think of it like Ant-Man - it's like a bullet, small and hard to target or avoid and extremely durable and seasoned Captains know how to use those strengths to it's advantage. Worf basically lived on the ship full time for a good stretch so he would have used that time to become an expect in it's combat capabilities. Throw in the cloaking system on top of that and it's little wonder it could take a beating and keep coming back for more - it was designed for that.

With the right ship and right captain anything is possible, and Worf and The Defiant were made for each other.