I think Hela said far more succinctly what I'm about to try and say here. (Well, said, Hela. 
)
We're good, Phantom. And as you can see, Big Jake and I have ended our debates with our customary comedic tendencies. So, I'm back on track with the original spirit of this thread.
I think, when it comes to the acting in a fan film, especially when it involves canonically (and iconically) established characters like Kirk, Spock, McCoy, Uhura, Scotty, etc., there are, for some, a base desire to hear the "voices" of those characters as portrayed by Shatner, Nimoy, Kelley, Nichols, Doohan, etc. again. It might not be apparent to some folks as to why fan actor portrayals might come "close, but no tranya". Some actors might be able to nail Shatner's halted recitation of lines, or perhaps even Scotty's brogue, but the "voice". There's that little niggling that one subconsciously might not be able to put their finger on when they're watching an otherwise top-notch quality fan film. It is a tall order to fill for the actors portraying these iconic roles. When doing a fan film based on the original Trek, yes, one wants to capture the spirit of the original series actors as much as within the fan actor's capabilities. It will be the same when more fan films are made of the Abrams-era Trek...the way those characters are portrayed by Pine, Quinto, Urban, Saldana, etc. It will fall on the fan actors to capture their spirit as well, if not quite their voices.
I'm gonna side bar here a bit:
It's almost like..... did anyone else, as a kid, ever get those little Read Along With The Record books based on their favorite sci-fi movies? Star Wars, The Black Hole, Star Trek III: The Search for Spock, etc?
When I was a kid, the only one I had was The Black Hole, and I had heard the one for Star Wars...more on that in a minute.
Thanks to YouTube recently, I'd been able to listen to some others of those kinds of records.
In the case of ST III, all it did was convey the story, and, if I recall correctly, only had one voice actor, and he was doing the role of Kirk. He tried so hard to nail Kirk's desperate side, but didn't quite hit it. (And the music and sound effects were like nothing I'd heard in the film version...it was just some melodramatic melody, and the sound effects were very generic).
I'd heard better impersonations of the main characters on those old LP story records that came with like 5 or six mini-adventures. (Although Sulu was made to sound completely nothing like his on-screen character in TOS... he sounded more like a caricature of a stereotype of just some Japanese dude who just happened to speak English, and who just happened to be helming the Enterprise.)
The Black Hole and Star Wars at least got the official music and sound effects down. The voice actors did decent jobs of impersonating some of the characters, but still, it was more caricature rather than approximation. (Not that it really matters to a small kid, unless he was born with the "nerd gene" --I would be one of those-- that enabled him to say: "That's not Vader!") And speaking of which, in the Star Wars record, yeah, Vader sounded nothing like his big screen version. Princess Leia sounded like an indignant twelve year old. Luke sounded even whinier. But, again, it still got the story (generally) across.
In The Black Hole, the voice actors did a better job of not quite sounding so much like over the top impersonations. They sounded closer to their characters, but I could still tell, even as a kid, that they were not the real actors. Still, the story came across pretty well.
(For better experiences, best to have simply gotten the LP recordings of the truncated versions of the actual movies. I used to have the TRON story LP, and listened to it constantly.)
Back to fan films:
That's where the other qualities of the film and the actors will aid in smoothing over that little speed bump... perhaps Vic Mignona is really good at copping Kirk's stance, and some of his vocal mannerisms, or someone playing Scotty might be able to nail his sly grin, or body language. The dialogue itself might very well channel the best of any character. The story might bring out those things in Star Trek that most resonated with fans and viewers of the actual shows. And visual effects and set design have far and away improved over fan films of years past. There are some really talented amateur CG animators and practical effects folk out there.
For some, those qualities might very well help "stick the landing", and some folk might feel as if they've seen a missing episode from Star Trek, or the Star Trek universe. For others, they are simply enjoyable experiences and efforts that, while set in the Trek universe, still don't quite come across with the impact of a full on, paid professional production.
Your actual parsecs may vary. Take these opinions with a swig of Tranya. 
TL;DR? Sometimes, it's all in the voice.
