When you've done her quest chain, something has happened to her. When you complete  , the quest text says: <The druid looks pale and her eyes stare blankly past you. She has left a package in anticipation of your return.>. If you complete the quest and talk to her again, the text says: <Thayreen's stare is blank. Cleansing Vordrassil's corruption has taken its toll on the druid.> . By working with the samples of slime you gave her earlier in the quest chain, and by cleansing Voldrassil and the Grizzly Hills, she has become corrupted by what corrupted Voldrassil (Corrupted by Yogg-Saron, as Ursoc's spirit says after cleansing him during  ). The druid is now silent and stares past people into the nothing.