Hey guys, I'll just add my two cents. This below is all my conjecture and assumptions and such that I culled from my mind and the internet, such as the MMKB, and other places. A note is that I'm assuming that certain elements that were put in probably for gameplay sake is true to the way their world works, like being in Human form in the presumably frigid weather of Area F and not freezing to death.
Reploid and Human desires aside, as well as their inherent differences (Manufacture (which is sort of creepy in my mind), vs birth). I'm wondering about the fundamental differences.
Reploids are still made, created in factories. Not really sure if they're born as children and their bodies are either upgraded as time goes on, or a pseudo-biological material allows their bodies to mature like humans so. Their minds also seem to have limiters that force them to learn like humans do, Andrew from Zero mentions that reploid children went to school with human children, and were better at sports and faster at learning, thus saying that unlike a normal computer which would only require one instance of an event to learn it, their minds emulate a human's a require at least a few repetitions to nail down a topic (This is assuming that Reploid science is still fundamentally the same over the course of 300+ years). Some of this explains how Giro, being only 15~25, is mature like a human adult, perhaps Reploids are made with developed minds, or learn at a quicker pace. Another theory could be that Reploids are created with adult minds, but different bodies which they can modify appearance-wise whenever they want based on their desire, but still keep their age programming.
Since Reploids posses programming on par with human personalities, they're capable of falling in love and possibly sharing the same carnal desires humans do. I theorize that Reploids posses sexual organs and sexual programming to facilitate emotional equality with humans, however, it's not known if Reploids have reproductive qualities of any kind, though this is highly unlikely. Also, apparently when a new Reploid is activated, they're also given a life-span (I'm assuming ~150 years as being the norm) to give them mortality. Humans cannot have immortality without sacrificing their unique trait, which is ironically their natural mortality. The Sage Trinity being a prime example as they're technically not human any more. It is still up for grabs whether or not Reploids receive anything organic in terms of modification, such as living skin tissue for their faces or something. Just being made and given a timer on your life would just suck for the humans who go through much more modification to be made equal.
Humans are still born as naturals (Cedre who sells E-tanks in ZX mentions to Aile that "Your parents gave you life"), which means that instead of being gestated in test tubes with a parent's genetic material, the parents still retain human reproductive organs with human children being born of the female womb. This means that the male genitalia and female genitalia and womb is left unmodified. At a certain age, perhaps around the end of adolescence, humans are given mechanical components replacing certain parts of their bodies, with their flesh parts being stored (For what reason?) in the Legion HQ. These modifications seem to include ear plates, probably linked to the brain in some fashion, and a fitting black bodysuit (Ciel the human is seen with one on at all times in the Zero series) probably for protection and hygiene and likely being their new skin, which would aid in healing from wounds and temperature control, especially in extreme conditions (How Aile/Vent make it through Area F without freezing to death). However in artwork for the Hunter's Guild, there is one hunter with his arms skin-colored, indicating that the pigment of the suit can be changed, or certain parts of the body can be left uncovered. The ear-plates and possible partial cyberization of the brain with implants make humans susceptible to turning into Mavericks through manipulation and leaving them susceptible to hacking and electronic warfare (Which is why Chronoforce's hacking "Timebomb" ability works on Ashe who is human). With all of these modifications, while biologically human, the humans of this era are cyborgs by definition, though by this time it's probably safe to assume that "Human" doesn't mean what parts of you are still natural, but rather how you came to be; i.e. born vs. made. Part of me wonders how a fully mature human natural (No modification to body) would be viewed in that universe. Like if some fully grown, unmodified adult fell through time and appeared in the ZX timeframe.
Truly the lines between man and machine have blurred to almost nothing.
Okay, I think I made enough rambling exposition there. I'm curious about how far the humans have been mechanized aside from the suits and ear plates. Vent and Aile can use Sub Tanks to recover "health" and use biometals to augment their physical bodies. I theorize that the only parts of humans that are replaced are the limbs, part of the torso, and certain important bone structures, like the spine, ribs, and hips. The reason I say this is because looking at the megamerged forms of the Chosen ones, every part of their body is covered in armor except for the full torso, which only has a breast plate over the rib-cage and their abdominal areas being uncovered. A reinforced body would also explain why the hero characters don't die in one hit from any attack, Grey/Ashe survived Prometheus' scythe attack while sustaining major damage in their normal forms. Human for Ashe, "Squishy" reploid for Grey.
I assume that nano-technology comes into play here when healing wounds in battle with health-items. Something that human bodies are probably capable to using in tandem with their mechanical organs and bodysuits, and something Reploids can do anyway because of their mechanical nature.
In the end, humans receive partial machine-bodies, with their flesh parts being held in cold-storage instead of disposed of (I can see a lot of limbs neatly cauterized in massive tanks, really creepy, eh? And to think Legion keeps those in the BASEMENT!). These bodies give humans longer lives and greater durability than ever before. I have no idea what Master Thomas plans to do with human body parts and pre-New Law reploid data as stated in the ZXA secret ending (Maybe a giant flesh-being with an immortal mind?)
On to my beef with Prairie. She's stated to have lived at least 200 years (post-Ragnarok Incident), and still appears youthful. Now, the Zero/ZX series has always been bad with lifespans, so we don't know how long she'd lived before you see her as Alouette in Z1. I assume her to be an old-type Reploid, one dating to before the new laws were implemented. Now, if she did comply with the new laws (She apparently DOES have a red triangle on her forehead like all new reploids which to me looks like an inverted Resistance symbol that's found on their old berets), she would have to have been marked and possibly reprogrammed with a new life span. Actually, I forgot where I was going with this... Sorry. Maybe she just was exempted or escaped the laws that gave Reploids mortality and is technically "immortal?" Maybe all she did was paste on a red sticker?
Last but not least, Reploid manufacture. Since Reploids are supposed to be identical to humans, each one has to have a different personality. I theorize that Reploids are made rather seldomly, or in small batches. I suspect its because creating a unique "Personality Matrix" or coding a unique Reploid "DNA" sequence takes time, as does making a new body with the high-specifications that the laws probably require. It also helps with population control and having a being with a complex mind makes them less vulnerable to turning Maverick, as opposed to mechaniloids who have basic programming that is easy to control.
On the topic of dying, I don't thing blowing apart is normal, just put in for tradition sake. They probably die by collapsing like most things do when killed by gunfire. Notice that only Pseudoroids and other boss characters explode. Except for Kraft who just went *fzzzzt* *dead*.
Any one have any thoughts on the Human's modification? Or anything else related? Sorry, this was the most relevant thread for me to post in. I hope this made sense, I just typed this out at once and lost my train of thought a few times.