I watched this vignette last Friday. Having had some time to stew on it, here are my thoughts:
I like the color palette used. I mean, it's a tad gimmicky, but it works here. Certainly it fits the ultimate purpose of the archway, at least as far as cultural significance and depiction go, so there's that.
As far as the acting goes, there's not much to say beyond actually letting your actors emote and react to their surroundings. I get that this was especially an effects heavy shoot/episode, but given what was (I assume) on the page, there should have been a little more gravitas to what the actors portrayed.
Then again, it's a short vignette, with really one set and only so much going on... I'm not sure what the impetus was for the away team to suddenly fire at the archway. They don't seem to spend much time actually investigating it, let alone consider the implications of what it actually is or even if it is what they think it is, what that actually means for the universe. Lastly, if there's enough evidence to suggest (as the characters seem to believe, given the final captain's log entry and the tag scene), why would these educated Starfleet officers believe, even for a moment that a little bit of phaser fire would solve the problem?
My last nit, and really it's just more a product of having lived with television production schedules for the last couple of years more than anything else ... the location shooting was great, but the vignette could have benefited from one scene on the bridge or with the Captain aboard. I don't know if that was just logistically impossible because of schedule conflicts, actor availability, or what not, but certainly pickups could be done the next time something is shot on the bridge for use later in a short like this.
Overall I was impressed. I like the concept of the story, and I think it poses some interesting questions, but the execution of how the characters respond to those questions was just a little lacking for me.
Looking forward to the next ep/vignette!