I think the main problem with the leviathans was that they made them too powerful. The writers had to pull out too many convoluted plot devices to keep pulling Sam and Dean out of the leviathans' grasp (deus ex machina).
I did like how they were able to mix in a lot of "monster of the week" episodes, though.
I wonder if a series finale story arc could involve the Winchesters finding God (literally, not spiritually). The angels keep implying that God is missing.