1) Well, so far, WotC hasn't revealed anything like an equivalent to Evasion for weapon properties. If they do, that's going to add a fair bit of complexity to the game.
2) Most monsters don't cast spells or use breath weapons - those are the exception. Most just have a melee attack and/or a ranged attack (sometimes with a little adder). And design trends seem to be reducing monster abilities and spells (compare MoM monsters versus Volos). TBH, I think monsters would be improved by having more mechanical heft to them.
3) Based on my experience, players will be unhappy when their melee character frequently needs to double move to engage without a save (thanks to the Slow property) or gets knocked unconscious (despite every attack missing their character).