Kevin Tran makes a significant return in "Trial and Error." After hardcore focus complete with considerable harm to his health (headaches, nose bleeds etc.), he's finally able to give the Winchesters the first big lead in their quest to close the Gates of Hell. It all results in one of the brothers passing a huge test. Mind you, this is the first of three challenges that'll need to be faced.
When the flashback portion of the season came to an end (still miss you, Purgatory scenes), I was kind of worried about what the second half of the season would bring. I had no idea where this show was headed other than the whole closing the Gates of Hell business. Then the Men of Letters story was introduced. The Winchesters got a home base of operations. Things started to open up in an encouraging way.