what exactly is supposed to be so special about those Holy Pandas anyway?
Nothing really. They became rare because they were made from halo stems in Invyr Panda housings and then Invyr Pandas went out of production leading to fever buying/selling, stoked by the web of lies, intrigue and internet drama surrounding its disappearance and miraculous return. Thus the myth of the Holy Panda was born.
The halo stem is too tall for its own stock Kailh housing - the centre post slams into the recess in the bottom housing. This is what causes the loud signature "tapping" sound of the switches when typing on them. This is also true of halo stems when transplanted into MX, Outemu Sky and Invyr Panda housings etc.
None of the holy meme switches I've tried bottom out correctly. The feel and sound is comparable to slamming the blunt end of a pencil onto a desk.
MX tactile frankenswitches fulfill what seems to be a longstanding demand for an in-production variant of MX Brown with a bigger bump. Zeal V1s did that initially but the furore around Holy Panda suggests theres demand for a bigger, longer bump than that, which may explain Zeal V2s departure from V1.
I still think Zeal V2s are better switches than Holy Pandas if thats the kind of thing you are looking for. They are actually designed using parts that fit each other so the stem doesn't prematurely smash into the bottom housing. The base of the stem cruciform sits flush with the top housing when the stem is fully depressed.
Exactly what is it that is physically different about the switches and what keyfeel does this translate into?
It works exactly like an MX Brown except instead of a tiny bump at the half way point, theres a very long and much bigger bump that starts higher up. In the case of Zeal V2s, it starts almost at the top of the key press. The bump is long enough that it can be felt somewhere around 2/3rds of the total key travel. Its really long.
If you visually inspect a halo stem, the legs are much longer than those on an MX brown stem and there is a long hill followed by a shorter valley. You can google comparison pictures to see what I mean. Zeal V2 stem is very similar to the halo stem with the main difference being the centre post is shorter.
Soundwise, halo frankenswitches are very different to MX tactiles because of the long centre post of the halo stem. They are much, much louder - almost MX Blue loud except theres no click mechanism. The tap sound is explained above. That long stem also changes the way the keys feel when bottomed out. In the case of halo frankenswitches (including stock halos and holy pandas), they never truly bottom out. You get this sudden shock of the stem post striking the base of the little pit in the bottom housing. If you type heavy handed, it can really rattle your finger joints. I personally find it unpleasant.