On second thought, I have an alternative solution to what it is lampshading, that is the broken suspension of disbelief that after stating that the terms of Quirrell's contract prevented him and others from investigating Quirrell's identity, Albus would leave the room, allowing his conspirators to investigate Quirrell's identity.
This theory sounds incredibly plausible from the perspective of the author wanting to use the lampshade trope, but from the perspective of the reader, that action was completely in-character for Dumbledore and doesn't actually break suspension of disbelief.
You can't say it's obvious unless you can point to something it is specifically lampshading. The best answers I've seen so far in this thread are that it's lampshading itself, in which case there's no reason for it to have been in the story. Traditionally when you hang a lampshade on something, it's something that the author needs as a plot device but actually wouldn't make very much sense if the story were playing out realistically, that is, it threatens suspension of disbelief. I don't think any of us would disbelieve that Dumbledore would have a strange vroopy thingy in his office, so lampshading itself doesn't make any sense (which I suppose would make it a meta-lampshade, which breaks the suspension of disbelief I have that the author is trying to use a trope in its proper context, and such abstractions could recurse infinitely).
The vrooping thing sounds like a centrifuge to me, though the pulsing light isn't something I'm familiar with in such apparatus.
If it is indeed a centrifuge, it would make sense that it was only mentioned -after- Dumbledore left the room. If they had somehow obtained a sample of Quirrell's blood, they might be separating it to do a DNA comparison against any candidates for his identity, which if I were HJPEV would have been one of my first (dozen) solutions to the problem of identification.
It's a reference to an episode of The Simpsons, wherein Lisa's boyfriend states: "I'm a level 5 vegan. I won't eat anything that casts a shadow."
Edit2: Primary source found.
Fair point, though that also removes the point of evidence that casting requirements are removed with practice.
"It takes a cracked soul to cast." and "Murder tears the soul." just says that if you've gotten to the point where you could cast it once, that particular pre-requisite is already accomplished, so the work to crack your soul is already put in. It doesn't say anything about removing the requirement of wanting someone dead.
Though, so long as we're looking at evidence, if we take Quirrell at his word, then his ability to cast the spell despite not wanting his opponent dead is pretty strong evidence that the requirement is in fact removed. In fact, we already know that some "requirements" to cast spells are not set in stone: from that same scene, Harry cast the true patronus without the carefully practiced stance and wand twitches, instead merely "one desperate wish that an innocent man should not die -"—but the constant requirement in this case seems to be the thought that accompanies the casting of the spell, which is why I'm hesitant to believe the wish of death is removed from AK's casting requirement.
In canon, Moody used the unforgivables on a spider, and given the prevalence of ostensibly non-sentient things-to-fear in the magical world (e.g. boggarts), it's conceivable that they could have found a particular magical creature that even the most PETA-supporting student would have no trouble excising from the world. Also, as far as I can tell, there's nothing in canon to contradict that curses' targets are limited to Kingdom Animalia (see also: Harry's existential crisis about sentient plants), and I seriously doubt there are any 7th level vegans at Hogwarts.
I interpreted the ease of casting the spell as a specific application of scope insensitivity rather than a change in the requirement to cast it. That is, while casting it the second time might be just as difficult (i.e. take as much mental/magical/spiritual energy) as the first, the third and fourth time would together be only as strenuous as the first, as would the collective fifth through eighth time, etc. It is already established in-universe that some form of personal mana depletion exists, and my idea of this difficulty reduction is an extension of that form of energy to the spiritual energy (established in canon w.r.t. horcruxes, dementors, etc.).
I'm a little surprised that HJPEV didn't immediately update his probabilities regarding Quirrell's motives in Azkaban with the new knowledge from Moody that "You've got to mean it. You've got to want someone dead, and not for the greater good, either.", which would seem to discredit the Defense Professor's excuse that "a curse which cannot be blocked and must be dodged is an indispensable tactic."
I dunno about yours, but my lampshades don't usually spin, particularly not with a "vroop".