>>299540 (OP)
>It seemed like it was leading to something more compelling, but they decided to ignore it.
It left questions about Pinkie Pie hanging in the air. While her immediate problem was resolved, it was clear by the end that she had emotional issues that left her prone to dangerously irrational behavior. Those issues were conspicuously ignored which made us regard Pinkie Pie as considerably less harmless than we had before the episode aired.
Guys who got on the ride in Season One will remember that up until "Party of One" aired Pinkie Pie had seemed singularly non-threatening, which was a big part of what had made "Cupcakes" amusing. The occasional harmless prank, as seen in "Griffon the Brush-off," was the limit to Pinkie Pie's maliciousness. She had even refused to prank Fluttershy over concerns that she was too delicate. We had accepted that benign playfullness as one of Pinkie Pie's defining traits.
"Party of One" turned that all on its head. She wasn't playing around with Spike when she held him to a chair and interrogated him, and the way that she chased Rainbow Dash down was considerably more threatening than the time that she had followed her everywhere with balloons and a heli-bicycle. Her cartoony powers no longer seemed quite so innocent. Then the visual representations of her wounded sense of purpose (the spotlight, the tea party, etc.) hinted at a new desperate and needy dimension to her character that had not been apparent before.
We never looked at Pinkie Pie the same way again, and really that was a good thing. Leaving that disturbed aspect of her intact at the end of the episode gave her a depth that she had not possessed before. It also set the foundation for her behavior in Season Two episodes like "The Last Round-up" and "A Friend In Deed."