I feel kind of arrogant in saying that the Blackfathom leggings are BiS on Three separate fronts. I apologize for what may seem contradictory, but let me explain.
First, as has been mentioned, the stamina is vastly superior. For this reason alone, BFL are worth taking over Silk-Threaded trousers, since much of this bracket as a healer is being able to tank enormous amounts of damage.
Second, the Critical Strike rating is something that scales with Restoration Druids extremely well.
Regrowth has a base 60% chance to be a critical strike, but, when you're spamming Regrowths to keep a target alive, that number often results in many non-crits. I aim to get about 15% critical strike in BG's so that my base crit chance is 75% on Regrowth. Increasing the consistency of the Critical Hits from Regrowth (and to Rejuve and Swiftmend, by extension) is a very powerful and often overlooked scaling mechanism for Druids that supercedes blunt Intellect stacking in my opinion. That being said, I don't make many concessions on my druid (who I would link if his server wasn't recently merged with Silvermoon, causing some unwanted problems with Wowarmory), since Stamina, Intellect, and Critical Strike are all extremely valuable.
Third, the armor difference between BFL and STT is a pretty substantial one. While not necessarily a make/break point by itself, there is certainly something to be said about reducing damage taken from Hunters/Rogues/Warriors.