In addition to the storytelling....for me, it is also a matter of visual scale.
Let's take Star Trek The Motion Picture for example. There were ten years between the end of the original series' run, and the premiere of The Motion Picture in 1979. During that time, there was the whole Phase II project which was aborted in favor of doing a big screen production due to the massive success of George Lucas' Star Wars.
When The Motion Picture was almost completed, the visual effects were pretty much unusable, with the citation that they looked no different from what was seen on TOS ten years before. With only weeks left before its big screen premiere, John Dykstra (who was the effects lead for Star Wars) was called in, and the effects were vastly improved. Also, the effects ended up filling the big screen nicely from end to end. The feel was epic, even if some felt the story was even more stilted than the worst episode of TOS.
The following five movies varied in feel. TWOK had the big screen feel. (I never saw it on the big screen, sadly, but I could clearly see its effect when it came to home video...and especially got released in letterbox format.) By the time of Trek V and VI, you could see hints of TNG in the set design, and in the visual texture. Although Trek VI retained its big screen feel, I could see that the television feel of TNG was starting to take over. (I think this is largely because the movies were still being done under the Paramount television division...not its feature film division, even though they were introduced as Paramount Pictures.)
When Generations was released to the "big screen", only a year or so had passed. The Next Generation was lauded as having visual effects that were close to feature film quality at the time. So, when Generations hit, despite a few minor differences, the movie really looked like an episode....and felt like an episode because it was written by television writers, not big screen writers. The first three TNG films were hindered by this. Everything was done with television writers and directors. Even First Contact, arguably the best film of the TNG series, suffered from the "television episode" feel. It's space battle was only about as epic as a fleet battle on Deep Space Nine, and did not fill the big screen very well. The gunfights on the Enterprise E also did not fit the big screen very well. I agree with the almost unanimous assessment that Insurrection really suffered from the "tv episode" effect, both in visual and storytelling terms. (This is also one of the reasons I was largely disappointed with the film "Serenity", which was the big screen leap for the all too shortly lived series "Firefly". It just didn't have the big screen feel because on a visual level, it didn't differentiate itself well from its television sire...and the story itself could've been a series finale or at least season finale. Serenity plays out much better on home video.)
Nemesis, arguably the worst of the NG films (although it remains my personal favorite) was led by a big screen director, Stuart Baird (albeit one with only three or four titles under his belt, who is better known as an editor), and written by a big screen scribe, John Logan (who, despite his Trek fandom, did end up applying a lot of "fanboy" vibe and some technical faux pas'.) But the action on a visual level filled the big screen much more effectively, and the space battle at the end (practically the entire last half of the film) looked and felt much more satisfying. Some of the acting also seemed, to me at least, a bit more in line with the big screen feel....not nearly as forced as is usually seen in a television episode.
Since this is largely a "first ten films" subject, I will leave the Abrams Trek films out, although I do feel that they certainly are worthy of the big screen.
This is only my personal assessment, but I hope it adds some to the discussion.
