Well you make valid points...especially number 2. As I've noticed that my hero who's the stand back archer is struggling with some armour types due to strength! But does that matter because he's the archer afterall and shouldn't be in melee, though he has been caught the odd time!
My DK is a tank of course, but the reason I choose that route is because when I look at the character model and the armour, I just don't like the look of him shooting a bow or casting spells! (I dont use magic in games anyway when it can be helped)!
My DK only looks good to me swinging a big pointy sword thingy - and overdosing on healing potions when needed! But I'm finding it's working fine for me...the damage he deals 200-300 (roughly) per swing is enough to kill most things outright, especially after my archer had a shot or two first!
And about point 3, yeah it does kind of seem the DK is slower than the hero, even when they have the same speed stat! Maybe the DK has more attack animation frames!?!?