OK, so in this game you play as a simple villager struggling to survive a world warped by thaumaturgic fallout after an alchemical war and populated by chimeric monsters.
This game rewards players for filling a role in a community and helping to defend that community both from outsiders and from the dangerous monsters that are used as livestock.

In this thread, I would like to get some opinions regarding character classes.
Classes are broken down by two dimensions (Role and Resource)
The four resources represented by suites of playing cards are:
Hearts= Water
Diamonds= Mineral
Clubs= Plant
Spades= Animal

The four roles are:
Runners (movement based characters used for scouting uncharted territory for resources and dangers.)
Collectors (specialists used for extracting resources from certain kinds of enemies.)
Processors (specialists used for converting resources into useable goods and sideline support.)
Maintainers (general purpose defensive and supporting characters.)

The sixteen core classes (as of now) are:

Runners:
Hearts= Sailors
Diamonds= Merchants
Clubs= Trailblazers
Spades= Riders
Collectors:
Hearts= Fishers
Diamonds= Miners
Clubs= Farmers
Spades= Hunters
Processors:
Hearts= Brewers
Diamonds= Smiths
Clubs= Carpenters
Spades= Butchers
Maintainers:
Hearts= Doctors
Diamonds= Bankers
Clubs= Guards
Spades= Pastors

My questions for you guys:

How do you think I could make each of these classes fun to use? What would be good realistic or over-the-top abilities to give them? Which ones do you think would be more fun to play? Which ones do you think would be less fun to play?

This seems interesting, I like the cut! I'm seeing a Japanese style console adventure game here, only tangentially realistic; something like Harvest Moon.

Can't comment on the question, though, as it's still totally up in the air how the play interaction works. Do you have some notion of what you do in the game or what it's supposed to do? Or do you just have this list of character classes and you want to figure what sort of game they would fit in?

Given the premise of the game, fighting chimeric monsters, the classes you've presented are largely irrelevant because they don't relate to the challenges they're presented with. There's three common ways to deal with this:

1. Change the classes to something relevant to the challenge: fighters, wizards, etc., like D&D
2. Change the challenges to something relevant to the classes or that uses the low-key nature of the classes to set the tone: building community, social struggle, or other simple-life and interpersonal struggles, like in Harvest Moon
3. Play off the contrast between the regular-people nature of the classes and the exoticness of the monsters: e.g. in Call of Cthulhu, where farmers and the like face elder gods

Me, I'm a sucker for the second option, I'll play the third now and then, and have become bored to tears by the first, but my tastes are not representative of the majority of the roleplaying market.The first option is the most popular by a very large margin, but it also means your game is competing directly with every other game like it.

My idea was that each of the classes could adapt the skills of their trade toward fighting monsters.
I mean, if you were raised from birth, by blacksmith parents, to pound sheets of metal into swords using a hammer and flame, you ought to be able to apply that knowledge to something's brains, right?

For Farmers I was thinking maybe they could use "fear effects", like building scarecrows, to heard large groups of small enemies into an enclosure where they can mow through them with weaponized farm implements like scythes. For an "ultimate attack" or whatever I was thinking that farmers ought to be able to set up an irrigation system so that they can "Open the Floodgates!" and drown lots of enemies using a powerful water elemental attack.

Butchers, right? They slaughter animals and carve up meat with their powerful knives. Ever see that movie Gangs of Newyork?

Basically, I'm looking for the tactical-combat of D&D4E except that instead of going on quests and raiding dungeons the characters are just trying to protect their community. In otherwords players in this game are on the defensive against monsters whereas players in D&D are on the offensive against monsters.

So, you seem to be going mostly with option #3, the desperate defense. That is, if you have the townsfolk be townsfolk, and not be given super-anti-monster-moves according to their jobs (pretty please don't do that). The desperate defense is a classic formula of post-apocalyptic action stories. It promotes a bit of creative problem solving and can generate a lot of tension.

Your setting might resemble the manorial system that dominated much of medieval Europe, where it was the job of the peasants to keep the ruler wealthy, and in return, the ruler provided a fortified stronghold to retreat to in times of war. Another variation might be the underground cities built by the Cappadocians in Turkey, built for the same reasons: http://www.hitit.co.uk/tosee/cappy/ucities.html