At what point does a series letting its hair down, so to speak, become too much of a good thing?
That's a good question. I think it edged on that in The Voyage Home, but certainly the Shatnerfest of V goes over the line. I do hope you guys watch Galaxy Quest, because that's the film that walks the line between silly and serious quite well. And First Contact as well.
Frankly, I like The Next Generation series (less so the film) because beginning with the second season, the series embraces the serious. A bit of serious science ideas mixed in with the silly and some serious ideas mixed in with the silly comedy. I think that's the balance I prefer. Over the top silly is great, as long as it remains true to the characters and have some solid ideas behind it. Much like the rule of fantasy: Remain consistent within the universe you've created but within that universe you can do whatever you want. The Voyage Home broke a couple universe rules and Final Frontier was the Star Trek universe through the eyes of Kirk, which no one asked for.