Broken Base: The fandom creamed this episode in a popularity poll taken in Doctor Who Magazine. Lowest-ranked overall out of any Eleventh Doctor stories at the time, and one of the lowest on the list, period.
In particular, the infamous leaf scene in the climax has been rather...controversial with fans. Some argue it's a powerful scene which gives Clara's character a chance to shine; others argue that it's a Tastes Like Diabetes ending (in an already sugary sweet episode) which completely undercuts the Doctor's epic speech moments earlier.
Continuity Lock-Out: The Doctor's casual mention of having a granddaughter threw some viewers unfamiliar with the Classic Series for a loop, as it was the first explicit reference to Susan Foreman in the revived series at that point.
Ham and Cheese: The story is incredibly cornball to many and very contrived with the leaf, but there's a beauty in watching the Doctor give an oratory assault on a flaming ball of doom. Even the episode's detractors point to Matt Smith's "Akhaten Speech" as one of his best performances.