As greg said, its the combonation of buffs and debuffs that make the minor things look big. Face it, traps stun you for 6 seconds, leaving your men unable to defend themselves for that long.
Healing Wards are dependant on Max HP, example Tauren will heal 26 hp/sec under the effects of healing wards. So basically you have to do more than 26 per second to cause "damage". Also, Purge removes enemy lust and other nasty buffs.
Lightning shield is terrible, especially if you cast it on something that can't move too well. You can practically fry an army of weak units. I have done this once vs a guy who had dotts and archers and killed most of his guys, hell I claimed a demon hunter that game. Truly nasty stuff my friend.
Also, Spirit Walkers and thier dispel makes things a little more annoying, plus if you dont have magic air or much magic attack, 1 spirit walkers can give 1 tauren 2 extra lives.