Lorica
"The most intuitive is to understand your audience's existing fantasy logic" - that's the issue right there. If you go by it, you are taking the easy way. Sure it works, but you aren't creating anything interesting. If executed well, is still entertaining. Personally, that's what I expect from PE combat/mechanis - familiar stuff executed well, with only minor unexpected things.
However, it's much more fun to create new logic (at least to me), subvert the comfort zone of the player - this is what I expect from TTON.
"Why should one do more damage than another in the internal logic of the game?" If you want talk mechanics then there are a lot of reasons you could use:
* better materials - I think AOD does this nicely. This is also consistent with real life - you could chop better with a steel axe than a bronze one, although either one can kill you dead.
* craftsmanship - things like balance and weight
* familiarity - someone who trained whole life with axe won't be as good with sword etc.
Anyway, the above is a bit of a digression.
"This is great if you have a great, exciting system." If your game doesn't have at least good system, then chances are it is shit. When I say "system" I mean how the combat, world, character interaction, etc. works together. Parts of that may be not as good as the combination of all of it, i.e. combat in PST is shitty but it is compensated by great narrative and interesting world, but it still must work and feel well together. With this definition of system, you could say that making a good game means making a good system. Hopefully we are talking about the same thing and not misunderstanding each other.
Going for hard realism is not a requirement, neither it is always desirable. Games are (at least to some degree) escapism, so 100% accuracy is not expected (unless it's a simulator like MS FS, Orbiter, etc.) Contradicting the real world by, for example, making things fall down in a curve instead of straight line, might be both something wrong, or great to do. It would be shit to do in a golf simulator set on Earth, but could be used as an indicator that you are on a space station that is creating artificial "gravity" by rotation.
As for chainsaw as a weapon - have you ever wielded one? It's damn inconvenient to swing around, not to mention it needs gas.
Human body is not a sapling-like thing. It varies in density a lot. It is one thing to chop off a hand, and another to chop off a leg or head. Muscles, tendons, skin, and bone fragments could jam a chainsaw since it's not designed to cut this kind of material. Look at the surgical saws - their structure is vastly different than a chainsaw. Surgeons also use different tools for cutting flesh and bone. Of course, some of the differences are due to not wanting to do too much damage, which, in case of zombie, is the exact opposite you want. I doubt that you could hack a zombie into pieces with a machete either. Maybe if it is really decomposed and all gooey (but how does it move then), but not in just a bit rotted state. A machete simply doesn't have enough energy in it (from swing) to chop off harder body parts - you may cut a hand, but would probably have difficulty with removing entire arm or leg. Cutting of the head would be slowed down by neck tendons and the backbone. There is a reason why weapons used for executions by beheading were heavy.
All this stuff can work differently in your game though. It would be actually fun to create zombies that are easily choppable and have this play a role in the game. If you do just that, and your zombie-source humans don't share this anatomy then you are making a crappy game because you now have it inconsistent.
Anyway, I hate zombies and like good discussions so do go on.