Again, TTEOTS wins out because the last episode of a Season or any show in general should always have a rather distinct feel to it. As in, feel like something different was done compared to prior episodes, or have something major accomplished by the cast. Or, in some cases, a much tougher obstacle/enemy crosses paths with the protagonists in the Finale of a Season/Series and serves to prove how far the protagonists had advanced based on how they end up dealing with this threat of sorts.
TGEA didn't feel like it accomplished anything major; it just felt like an ordinary episode with a few characters from the movies returning to guest star in the episode. We've had plenty of episodes like those already, so it was familiar territory. TTEOTS, on the other hand, is the ONLY time we get to see Spike having actual "dialogue" because of the fact, as the episode title suggests, we see things through his viewpoint and thus his thinking gets translated into dialogue outright. As a result, the episode felt like it accomplished something major; by giving Spike an actual major speaking role outside of a single word only moment of dialogue like in Movie 4 or Movie 8.
It still doesn't really qualify for what you mention makes a season finale.
I don't get why people don't like TGGE and don't like it as the season finale. Am I really alone in this boat, here? 
I literally just said in that post why it qualifies for a better season finale; Spike, a character who normally never has a major speaking role, actually has his first and thus far only major speaking role in that particular episode. It's something that normally you would only expect a finale of sorts to do; something big that hasn't been done before in the prior episodes.
Let me give a similar example from another two season series. In DinoSquad, Ms. Moynihan is revealed in the first episode to be a
Velociraptor/Human shape-shifter much like Victory Veloci himself is. However, throughout the series, we never see Moynihan transform at all, even though she's had a Raptor form designed for the show proper which was used in a flashback for her and Victor's origins.
In the final episode of Season 2, and thus the final episode altogether since the show has ended, the five members of the DinoSquad have all been subdued and are unable to escape Veloci's henchmen due to an ambush. Moynihan, arriving on the scene and realizing this, finally transforms much to her chagrin since she doesn't like the idea of having to use her Raptor form anymore. As a result, we finally see her in the present day as a Dinosaur, and we have the first direct conversation between her and Veloci detailing a bit more on their past with each other. She's captured, yes, but she succeeds in freeing the other five so that they can free her by the end of the episode.
So, in short, both episodes did something with one member of the main cast that, up until that episode, had never been seen before prior in the context of the overall series they're from.
