These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Growing the Beard: While the quality of the series varied considerably each episode, the series started started to become consistently good from 'Angel of the North' onwards.
Tear Jerker: The final scene of the finale, as K9 very nearly dies. Particularly saddening is Darius sheer denial.
Darius: But he can self-repair. He's done it before!
Professor Gryffen: Not this time.
Darius: But we always win! K9 always wins, that's how it goes!
Starkey: Not this time.
They Changed It, Now It Sucks: There is a lot of divisiveness created simply from altering K9's original retro design to the new one. Fans are very split between whether to like the upgraded tin dog for his enhancements or equate it to making K9 appealing to the new generation. K9's design has been very consistent since the seventies, and it isn't so much the fact that he's changed more than the fact that his spin-off is about as divorced from Doctor Who as you can get. (The BBC isn't involved, so the show can't use any other Who characters.)