the story: Troi loses her empathic ability.
similar to: "Broken Link" (Deep Space Nine), "Mortal Coil" (Voyager)
my thoughts: Of all the episodes that centered around Troi, this is probably the most important, the essential one, where her whole character is deconstructed. Like taking away Odo's shape-shifting ability ("Broken Link" and half the episodes of the fifth season) or Neelix's will to live ("Mortal Coil"), taking away her half-Betazoid-derived powers is about as big as it can get.
Naturally, she's completely devastated, and the episode is a complete examination of her coping with that, and how she fits in with the rest of the crew, with or without her empathic ability. The result is about as fascinating a character study as Next Generation ever did, even if it's wrapped up in a fairly generic sci-fi story that is otherwise undistinguished (don't blame the series and/or franchise if you don't like that sort of thing; it's pretty much half the reason Star Trek exists at all, something a lot of people have a hard time understanding).
What results is a version of Troi who's more significant than perhaps she is otherwise, in the series or later films, someone who has unique insight not only into the minds of others but the psychology of the situation. This would have been fascinating to explore later, but Troi reverts to what she always was, which is a real shame, and somewhat diminishes the impact of the episode.
On its own, "The Loss" is a classic allegory in the best Star Trek sense, what might almost be described as a PTSD episode (Deep Space Nine's "It's Only a Paper Moon" explores this in more detail, and more directly). The fact that it features Troi and augments her character, however briefly, is kind of a bonus.
If you're a fan of Troi, this is the one to watch.
Whoopi Goldberg (Guinan)