Can someone post a description of each lieutenant's plot deck, what they do to the game and how difficult they make the game for the heroes / difficulty mastering the plot decks?

Here you can find the Plot Decks of all (except for Skarn) Lieutenants aviable. I have Skarn but I'm not at home atm, when I come back I'll take a pic and show it to you.

As for which make heroes cry, I would say that depends on what you are looking for: do you want to summon a big monster in play, then go for Gargan, Queen Ariad or Skarn; do you want to use the cards from the Plot, then go for Belthir's Deck, Raythen (for Treasure Hunter counterpick) and if you want to summon little useful Lieutenants go for Splig and so on. I mean, I don't own all of them so I can't tell, but yeah, I think you should ask those three questions before looking for a Plot Deck