I do believe that the 20% damage reduction for armor (right branch of the Defense tree) is a real(!) percentage and not slapped onto your existing damage reduction as percentage points.
At least I see that with my crusader: 23 damage reduction on his armor does NOT go up to 43 with this skill, but only 28 (in the summary on the character screen).
The Math: 23*20%=4,6 ---> 23+4,6=27,6 rounded to 28.
Nice thanks for the video
Nice work i second that 100% same thing i was talking about with some cabal members :).
Another thing in defense early game for the melee people is to instead of taking the two points on the left, go up one and over to the left one on the right hand side. That gives a flat 2% damage reduction.
You are right about the Assassin though, kinda squishy in comparison. The two percent does help add a little bit of mitigation to the melee Assassin though. Still fun, just not climbing the 150+ difficulty of the Juggernaut (as long as it's not the AoE nightmare of rebels...lol).