"Observer Effect"
This is a much better take on alien possession than "The Crossing". Immediately it makes the premise interesting by presenting the aliens as non-hostile and even likeable, showing us a legitimate "outsider perspective" from explorers of a different kind (which is what the Wisps claimed to be, but none of us were fooled for a minute, I assume). It also presents the temporary theft of bodies as a natural and non-malicious means of research for them, akin to listening in on someone at the keyhole - not exactly upstanding, perhaps, but hardly questionable or disturbing. Clearly as far as Organians are concerned they're not doing anything wrong or causing undue harm, so who cares? It helps make them alien and so adds legitimacy to their considerations of physical beings' responses. The possession scenes are fun and it's a relief that we're clued in from the very start rather than wasting time on a pretend mystery.
While some of the pair's often simplistic moralizing could have been annoying, I think the episode largely (not entirely, but largely) avoids that pitfall due to the Organians being of such truly alien origin. What they're saying comes from a vantage point beyond the corporeal experience and therefore potentially completely removed from anything Human, so we can't really make conclusions as to how far they've had to come or how complex and counter-intuitive their perspectives on what they discuss are. They're enigmatic, but in a way that raises interesting questions rather than being annoying. Again, contrast with the Wisps, who drove me half to distraction as I tried to comprehend what the "rules" were for their existence and abilities.
I like the use of an alien biohazard as a plot device, to underscore the dark side of interstellar exploration, where each new world is a danger as well as a wonder. Here, the threat is alien in every sense, because the pathogen isn't native to the biosphere it's discovered in. Making this site of contamination the focal point for alien study (even if it's a rather unorthodox study) is even better. It all contributes to a sense of the galaxy as a lived-in place, with all sorts of fascinating, low-key scenarios to explore where worlds and species and phenomena have acted upon each other. When Humans head out into the galaxy, we want a sense that the galaxy's been existing as a dynamic place long before they arrive.
The downside - really the only downside to what is otherwise a memorable and well-conceived episode - is the slight (rising to uncomfortable near the end) Humans-Are-Special vibe. The idea of the Organian observers having a lesson imparted to them is fine, the idea that they're surprised by Archer's actions and responses is great, but the suggestion that Humans are somehow different from everybody else leaves a sour taste for me.
To be fair on this point, there is at times the suggestion that the mindset Archer and co demonstrate is not so unusual after all, or wouldn't be had other subjects gotten to this stage, only Reed-Organian won't see it (Witness "the Cardassians showed similar compassion for one another", which he dismisses simply because of the final decision to contain the infection by abandoning the sick crewmen). The point is made that very few of the observed species ever got to this point so logically they wouldn't have had the opportunity to demonstrate such selfless compassion. Room for protest regarding the conclusions is implicitly acknowledged, I suppose. The Organians are clearly not inclined to speculate on such things unless they actually see it unfold, but still...
Still...
You never encountered a race with both the medical technology to detect this thing and the natural compassion or reckless heroism to do as Archer did (or something equivalently eye-opening)? You never encountered Xyrillians or Axanar or Trill or Denobulans, all of whom are clearly strongly social, compassionate and scientifically inclined? Not Deltans or Risians or other cultures like these where easing of distress and emotional comfort of all description are intrinsic to social interaction? Not Andorians or Tellarites or other races like these who never let little things like pragmatism or "common sense" get in the way of their passion, including their passion for one another? What of the Vissians, who routinely have married couples serving aboard their ships? What of the Xindi-Insectoids who collectively chose to suffocate so their eggs would be safe in "Hatchery"?
I suppose this incident might truly be the perfect storm - compassionate culture, suitable medical knowhow, ship with resources and facilities good enough to attempt containment and cure - but wouldn't those factors then have come up in conversation? Okay, I doubt Reed-Organian would note them because he's rather welded to his existing conclusions - "all physical species are the same" - (then again, he didn't seem that bad - dogmatic and inflexible, yes, but not a truly closed-minded thinker), but Mayweather-Organian would stress that the comparisons were laid across too many of these varying factors to be truly fair, surely? A point he might succeed in convincing his companion on, since the uniformity of the test is seemingly important to Reed-Organian.
There's also the fact that the whole thing is undermined a little by the fact, actually a plot point at times, that Archer's doctor is not only non-Human, he's using non-Human medical equipment.
Hooray for Humans! (Though a Denobulan and a Vulcan do all the hard work...)
On a similar note, no-one even bothered placing a warning beacon before? No-one? Was Archer really the first person to order that? The Organians wouldn't have destroyed any beacons left there - that would be interfering, and they're not hypocrites from what we see. Indeed, Reed-Organian even complains that they can't continue with these studies now that there's a beacon.
There's also a possible conflict thematically with earlier episodes that cover the issue of responsibility for those you have the power to help but whom you find yourself in a position of detached superiority relative to. After all, what would the Valakians of "Dear Doctor" say about Archer's decision to withhold the full possible degree of help from them? Are not the Organians just doing what he did in that situation - basing the amount of interference and aid they're willing to give on a difficult but considered judgement of the appropriate degree of responsibility to another culture, and the probable developmental pattern and considered maturity of the subject population, no matter how subjective that might be? It's not quite the same, of course - I'm not suggesting a direct equivalence - but there are definitely sizable overlaps. I'm not even criticizing the possible (probable?) hypocrisy, because this sort of multi-tiered perspective and the difficulty in applying a unified standard is often very rewarding to consider, and understandable in a universe where the division between species and nations works on different levels (warp powers look on non-warp powers as "ascended" races look on warp powers). Plus the whole idea of the silicon contamination in the carbon biosphere serving as a focus for Organian testing is even suggestive of the whole "outside, alien contact (contamination?) offering opportunities for learning as well as danger". I don't care if there's an inconsistency or a note of hypocrisy, but without room to address it, it makes me ever so slightly uncomfortable.
Still, don't get me wrong - this was a great episode, it really was; I just think it needed to tone down the Humans-Are-Great vibe to be truly at the top of its game. Emphasise more the "Organians have lost sight of a few things" angle rather than the supposed "Humans are better" angle (though the Organians were never vilified, which is nice). Still, good show.
First Appearances of Things That Are Important:
Organians, although the fact that they're Organians doesn't mean anything; they could have been any non-corporeal race. We don't learn much about Organians, other than that they're a species who were once corporeal, are now incorporeal, and that they interpret the change as positive development. When we meet the Organian mainstream, in a hundred years' time, we'll discover that Reed-Organian is actually somewhat of an eccentric himself, actively studying other species rather than ignoring them completely. Which is interesting - and will, perhaps, help us put Mayweather-Organian in perspective. The other hippies look on him as a hippie.
Silicon-based life, if a virus-analogue counts as life, which is questionable.
The Cardassians, previously implied for those in the know due to the interstellar market for tojal, are named for the first time. They're nobody important... yet.
Next Time: "Babel One".
But it is a good episode, I agree, and I get what you mean by "familiarly Trekkish". Given how much time I spent on this one complaint in my post up there, it might look like my objection was a far bigger deal for me than it actually was. It was really just a minor point of discontent; the episode is a strong one, and enjoyable. I'm just nitpicking.
My comment quoted here was an observation, not a complaint. And I agree that giving an established race what has to then be interpreted as a dissenting cultural offshoot is always welcome (even if it's only accidental). 
