By what reactions do moisture and light steal away my coffee's freshness?
There's a really nice write-up on this topic on Black Bear Coffee's website (which Aaronut linked above). 

Though it's not mentioned in your question, oxygen is actually the first culprit in loss of freshness:


  Separation from oxygen has been the primary strategy, with good
  reason. Oxidation obviously contributes significantly to flavor
  degradation and loss. Ambient air contains 19-21% oxygen and only 14
  cubic centimeters of oxygen (or 70 cc of ambient air) are enough to
  render a pound of coffee dead stale....A common myth is that coffee is not able to take on oxygen immediately after roasting due to carbon dioxide degassing. However, Michael Sivetz estimates that instead of 21%, about 10% oxygen surrounds degassing coffee –certainly enough to initiate oxidation.


The article doesn't specifically mention the effects of light on the freshness of coffee, so I would assume that light's role is mostly related to increasing thermal energy.


  The common thread in all deterioration processes is thermal energy.
  The rate of staling will be a function of the thermal energy applied
  to the coffee and how it is distributed. An important mechanism of
  thermal energy distribution is moisture. Roasted coffee will also
  absorb water at any time it is exposed to humid conditions, especially
  in the presence of high temperatures. Water quenching can add
  additional water and some of the deterioration processes themselves
  create water as a by-product. Within whole bean or ground coffee,
  water will take one of two forms: free or bound.
  
  "Free" water is mobile and can increase staling processes by retaining
  and delivering thermal energy and oxygen to the aromatics, acids, and
  oils, or bringing together sugars and protein to initiate
  non-enzymatic browning. "Bound" water (bound to surfaces) is not as
  mobile or available to solvate reactants. The ratio between free and
  bound water is called "water activity." It is increased any time the
  coffee comes into contact with humidity or high temperatures ("bound"
  water often becomes "free" water upon heating). A relatively low
  ambient humidity of 25% can cause roasted coffee to increase its
  moisture content to 5%, with water activity also increasing. Lipid
  oxidation is accelerated at heightened water activities, but is not
  usually measured in coffee, despite its effect on freshness. Studies
  show that a water activity ratio of above 0.5 contributes
  significantly to increased rates of non-enzymatic browning and lipid
  oxidation. More studies on water activity and its relation to coffee
  freshness are currently being conducted.


"Freshness" does indeed appear to be a subjective term, so I'm not sure there's a canonical definition about the chemical components of freshness. The Black Bear does provide an example, though: "Coffees known for their delicate and sweet aromas (such as certain East African coffees) depend on aldehydes for their unique flavor and are not good candidates for open bins or ground sales."

Other sources allude to the chemical components of coffee's taste but do not always enumerate them or distinguish "freshness" from the overall "coffee"ness. Here's one such statement from a reprinting of an article that appeared in Chemical &amp; Engineering News:


  A thousand volatile compounds have been identified in coffee, though
  just 40 or so of these substances "have been demonstrated to
  contribute to the alluring smell," Hofmann noted. They include
  β-damascenone (which has an aroma like cooked apples), 2-furfurylthiol
  (sulfury, roasty), 2-isobutyl-3-methoxypyrazine (earthy), guaiacol
  (spicy), 2,3-butanedione (buttery), and
  4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like).
  
  The flavor and aroma compounds derive from multiple chemical
  reactions, including the Maillard reaction, caramelization, polyphenol
  degradation, polymerization of carbohydrates, and pyrolysis.


The closest I could find to someone identifying the chemical compound responsible for "freshness" is further along in that article:


  "Unfortunately, the pleasant fresh-coffee aroma cannot be simply
  preserved," Müller said. Once again, it's the sulfury-roasty aroma
  quality that suffers during storage of coffee beverages. "This is
  mainly due to the decrease of the coffeelike-smelling compound
  2-furfurylthiol (FFT)." [said senior scientist Christoph Müller.]


The findings of this article reiterate Black Bear's claim that water activity is responsible for loss of freshness, and these processes are actually determined at the time of roasting as much as the storage conditions you have after you purchase your beans.


  Once beans reach the desired degree of roast, they are cooled rapidly
  with air or water. Air-cooled coffee beans contain just 1–2% water,
  while water-cooled coffee beans contain as much as 5% water.
  Baggenstoss studied the effect of the beans' water content on the
  stability of flavor compounds during storage. He found that aldehydes,
  pyrazines, and diketones such as 2,3-butanedione were unaffected by
  bean water content.
  
  On the other hand, compounds such as dimethyl trisulfide formed faster
  and reached higher levels in beans with higher water contents.
  Dimethyl trisulfide is formed by the oxidation of methanethiol, which
  is broadly related to the perception of coffee freshness. "Therefore,
  the coffee with higher water content seemed to lose fresh attributes
  faster than air-quenched coffee," Baggenstoss said. Furthermore, "some
  of the impact compounds are more rapidly degraded during storage of
  coffees with higher moisture content."