Originally Posted by
Endless_Legend
Important thing here: Getting killed by a knight in full plate is not a knock against Syrio. It means the full plate did it's job, and the knight was competent enough to attack and defend such that a guy with a stick isn't able to use it to kill him.
Plate armor is not a disadvantage in any kind of combat, including a sword duel. It is still an advantage. You're basically invulnerable to light weapons like rapiers (or whatever Syrio would have used) as long as you're reasonably skilled enough that you never, ever give them the oppurtunity to jam it into one of the armor's gaps.
A moderately skilled swordsman with a longsword and full plate should be *expected* to defeat a vastly superior swordsman with no armor and a dueling weapon. That's how real sword combat works, and that's how much difference wearing armor makes.
Note: I'm not saying Syrio wouldn't have killed him, he still probably would have if he'd had a real sword. But that only further shows how much of a master Syrio was, since doing so is the equivalent of defeating an enemy army that is entrenched, has the high ground, superior numbers, at night, in the rain, at the intersecting corners of four mapsheets.