I love this one. I enjoy really early Trek, before all of the personalities are in place and the sound effects ironed out. There's an eerie quality to the show, maybe because the scripts were more sci-fi slanted, before the humor and camraderie clicked on, and also because of the music. At the same time, the ship felt populated by real paople and not just extras.
The Man Trap is a good example of all this. Spock is still a cold bastard for the most part, but reacts more emotionally than later; Uhura, Sulu, Rand, and a few guys in the corridors all have lines. They don't do a lot to push the plot forward, but it does add solidity to the crew; when Spock expands his search radius at his console, there's a great electronic, retro 50's style sci-fi sound effect that I always loved. Way out from what we normally got (and I'm sure Roddenberry wanted things to sound more realistic), it's a nice fun detail; Kirk is mostly business and his friendship with McCoy is barely evident - but Kirk does get a great humanizing bit with the chili peppers. A very cool illustration of Kirk and his friends outside the ship. Not everyone out there helped him get into the academy, or was a hero. Just a bunch of captains.
Lots of great goofs due to the extremely rushed schedule for a show nobody was used to making yet. A number of early episodes have goofs that we find, but weren't caught or were deemed no big deal by airtime. Adds to the fun (when I came aboard!).
As far as the episode itself, it's one of my favorites as well. I revist this one a LOT. The story is really good, and the "monster" isn't the typlical Irwin Allen stuff, it's more in line with The Outer Limits. Early Kirk wants the damned thing killed and he has some very cutting, and what we'd later call "non-Kirk-like", dialog. But it's real! Picard would have sprinkled little nuggets around to lure it into a humane trap. Kirk, while probably willing to leave it on the planet, is protecting his crew. And since the kill setting took two long shots to work, I assume stun would have had no effect.
And, come on! The best scene in the episode is Spock double-fist hammering Nancy something like 7 times before she whales him across the room with one blow. It's proto-Spock at his best. And it's exciting as hell.
The guest cast is great, everyone is pretty darned good in it. Except Gertrude. That damned glove plant... Even as a kid, I thought that stank. But it was another illustration of "strange new life." I give it a pass now, the sound it makes is awesome.
The score by Sandy Courage is outstanding. Very haunting, eerie, and more electronic than would be the norm for Trek. Marc Daniels keeps it all moving.
I usually prefer Where No Man as an opening episode, but I can fully understand why NBC went with Man Trap, even beyond the reasons Justman and Solow gave: the cast is well represented, the story is sci-fi and not for kiddies, and the ship feels populated by real people. And there's a great monster. I love monsters generally, but this one is really well done and frightening.
Not the best episode, but a favorite. Top marks.