First, a quick fun fact: we currently don’t have any good reason to call the up/down quark generation the same thing as the electron/electron-neutrino generation. That is, the leptons and quarks both seem to have three generations, and each generation seems to have different masses, but there’s nothing that lines them up with each other. Thus, they are always ordered according to mass (due to a lack of anything better to go on at present).
As for any layman-noticeable differences due to a lack of heavier generations: absolutely! A few off the top of my head –
Since MikeS mentioned the early universe above, I’ll start with a follow up there. Yes, the number of particles species present, and their masses, affects cooling processed as well as (and perhaps more importantly) the clumpiness of the resulting matter distribution. Killing the heavier generations would have huge effects on galaxy size and formation rate. In turn, this would affect star size and lifetime.
But, if we quietly assume we make it through the first dozen billion years or so for the sake of discussion, then…
Without the heavier quarks, the masses of the proton and neutron would decrease by something around 1%. (This is a remarkably difficult thing to calculate, and predicting hadron masses from first principles is an active area of research. But 1% is the ballpark.) This means that everything would be lighter by 1%. Your weight as read by a standard scale would drop by 2%, since the earth would be lighter, too, making gravity at the surface weaker by 1%.
Relatedly, the mass differences would alter the energy levels of nuclei. Things that were stable could become unstable; things that were unstable could have different daughters; decay times would change; etc. It would take some effort to figure out exactly which changes would matter, but I suspect many would be significant to life.
The absence of heavy quarks (especially strange quarks) would alter nuclear magnetic moments, which would in turn have similar affects to the mass change on nuclear energy levels.
There would be drastic changes caused from altering the cosmic ray flux. At present, there are lots of high energy particles (mostly protons) hitting nuclei in the upper atmosphere. These collisions produce pions and kaons, and these particles subsequently decay, producing a variety of things but in particular copious muons. The muons (and some of the un-decayed parent particles) pass through the atmosphere and rain down on the earth’s surface.
This process would change in two catastrophic ways. First, there would be no muons, so any muon-driven process (see next paragraph) would cease. Second, the pion lifetime would grow tremendously. I estimate around a factor of 8000 increase, since its primary decay mode (to a muon and muon neutrino) would be disallowed to to non-existence and the alternative decay mode (to an electron and electron neutrino) would still be heavily “helicity suppressed” (which is outside of the scope of this post). In any case, this would mean pions would be able to survive all the way down to the surface instead of decaying in the upper atmosphere. (The more obvious issue that there could be no kaons produced in the primary interactions is dwarfed in importance by the above items.)
Two major effects (there would be numerous minor ones) –
(1) It is speculated that the muon flux at the surface is a driving force for genetic mutation, both from direct nuclear rearragnement and from the radionuclides externally produced and subsequently incorporated into the body. While that process would cease, the new (and greater) bombardment from pions would bring havoc. A pion is hundreds of times worse, radiation wise, than a muon at the energies of relevance here, as it tends to interact via the strong force, inducing showers of daughter particles each of which does its own nuclear damage. I suspect that a lot of life would just die out, actually, if it was a sudden change. Evolution might have been able to handle it just fine over the eons, but who can say?
(2) Cloud seeding is greatly enhanced by cosmic rays, especially pions. Things would become a lot cloudier and, thus, colder (easily cold enough to freeze the planet, I’d suspect).
Supernovae: isotopic abundances on earth would be affected by changes in past supernova explosions induced from the lack of multiple neutrino flavors. Neutrinos produced in the initial nuclear reactions would be unable to change flavor and, thus, would have a net higher interaction rate with the stellar material. (These interactions are a principle driver of the explosion process.)
It is perhaps worth noting explicitly that a layman would notice changes in isotopic abundances via their effects on industry and the economy.