Before we discuss the effects of shoeing, let's look first at the advantages of shoeing: (you will understand the significance of these statements if the horse is your priority!)
1. The ability to use a horse on any terrain, at any time, without giving any thought to providing the  
   horse with a proper lifestyle or hoofcare 
  * But at the expense of the horse's health and life expectancy *
2. The ability to temporarily ignore the biological limitations of the horse
   * But at the expense of the horse's health and life expectancy  *
3. The ability to make a lame horse useable for a while longer (during which time the damage
    continues to worsen)
  * But at the expense of the horses's health and life expectancy *
Remember: Horses were born with all the footwear they ever need.
The reason to shoe a horse is to use it beyond its biological limitations.
The Effects of Shoeing

There are a few  very common statement made by horse owners. Some may sound familiar to you, because you may have said or heard something similar at some stage or another in a discussion about "shoeing or not shoeing"...... for example:
"MY horse needs shoes,
because.... is a performance horse has brittle hooves has shallow hooves is a thoroughbred
   (a warmblood/QH/arab...)
....I ride on roads farrier / vet / instructor
     said so...."

Upon deeper analysis of each and every reason for shoeing, we always arrive at the same conclusion:
Any living organism has biological limitations.  Horses have them too.

Human domesticated horses about 5000 years ago.
Non of the early documentations about horses speaks of any form of footwear
Xenophen (1350BC) writes about "toughening the hooves", ancient Hittites, the old Egyptians rode/drove over great distances  no mention of any kind of hoof cover!
There are no artefacts that would prove any existence of iron horse shoes before the Middle Ages!

The Dawn Of the Horse Shoe: The Middle Ages!
If and Why is this a coincident?
Noblemen and Castles come to mind, Knights in heavy Armor.
Horses would have to be kept in close proximity to these warriors or noblemen: Within their castles. Castles were strategically build on elevated positions (on hills, above villages/towns). There is no room for paddocks within the walls of castles. Horses were kept in stables complexes.
  (Taking away some of its biological need for health - as discussed before)?
  We weaken the organism.
  (This is what forms when urine/excrements are exposed to air).
  It weakens the organism.
  Ammonium is present in stable beddings and in the air.
  Ammonium dissolves protein: HOOF HORN IS PROTEIN!
(also, ammonium vapour mixed with stable dust is the reason why many horses, even today when conditions are      "clean", suffer from  respiratory problems)
      Lack of movement   *   Exposure to Ammonium   *  Unhealthy Posture   *
      Lack of moisture (water)   *   Drying effect of bedding

would have caused deterioration of these horses' hooves.
A protection, something that kept the hoof "together" and kept the horse useable, needed to be developed!)
An iron rim, nailed onto the hoof became the trademark of noblemen!
All this, because it was more practical to keep the horses within the castle boundaries than keeping them in paddocks below in the village!
Today we still believe, horses "need" shoes to keep their hooves together (and to protect them from wear etc). All this belief is based on those who misunderstood the reasoning of the noblemen up in their castles and wanted to have what they had. The common folk associated (and most of us still do) the "clip'clop" on the pavement as a romantic/positive sound and we consider a horseshoe to be a sign of luck.
Not for the horse, unless it has lost it...

The term "SHOE" is also very deceiving (in German the horse shoe is called "HUFEISEN"  which means "hoof-iron".  And that is what it is  a shaped iron(metal/plastic) rim, nailed (glued) onto the natural hoof of the horse.

Gradual deformation of hoofcapsule
Shoe is nailed on when hoof is lifted off the ground, therefore it is fixed in its most narrow contracted state
Contraction, pain, changes in movement, tripping, muscle problems, joint ossification, arthritis, deformation of coffin bone, lateral cartilage, damage to corium, set up for coffin bone rotation, laminitis, thrush, navicular syndrome, white line disease
Destroys, weakens hoof wall through nails
Perforation, structural damage
Desiccation and loss of elasticity, insulation of horn capsule is breached, reduction in metabolism due to temperature drop when cold, effecting laminar horn production and therefore coffin bone suspension
Reduced circulation
Hoofmechanism can not function (reversible deformation of hoof capsule)
Overstressing heart, circulatory problems, metabolic problems
Metabolic disruptions
Hoofmechanism can not function (see above)
Protein imbalance in the system, skin, kidney/ liver problems, colic
Changes in weightbearing, breakover and limb movement
Due to shape, weight and properties of shoe
Muscle and tendon problems, sidebone, ringbone over-reaching, self injury
Metal vibrates when struck by something hard.

Degeneration of capillaries, tissue necrosis, pathological alterations of corium tissue, chronic numbness, coldness (Raynaud's Syndrome)
Microfractures, ossifications, tendon problems etc.

Unphysiological stresses on hoof capsule
Tension through fixation
Horn cracks, white line separation, bruising, keratomas
Impaired shock absorption
Hoofmechanism can not function (60-80% of shock absorption is lost)
The jarring of the horse's unshod leg cantering on pavement is less than the jarring of a shod horse walking on pavement.

Ossification, joint damage, arthritis
Greatly reduces sensation of the ground
Reduced nerve function (reduced circulation)
Danger of mis-stepping, bruising as stones can be higher than the rim of the iron
Increased weight of hoof
of shoe
Increased centrifugal effect (altered gait), ligament sprains, increased impact force and consequent damage (to horses and human toes).
Changed traction
Either too much or too little due to metal surface
No suction effect on smooth/slippery/wet surface as bare hoof has, unhealthy resistance when turning. Joint, ligament, tendon damage, ossifications
Conformation changes
Pain or trimming errors cause horse to seek more comfortable positions which could result in joint adaptation
Crooked hooves, joints, coon, bucked knees, cow hocked, sickle hocked, "offset" knees, hunter's bump, out behind, base narrow.. etc etc.
Prevents development of healthy coffin bone in young horses
Horses shod before they are mature (most TBs) have not yet fully developed their palmar processes
Narrow coffin bone = contraction
Damage to trails, roads, pasture
Iron shoes "plough" ground, destroy pasture and trails. Damaged flora
Increased restrictions to riders as horses get "banned" from great riding country.
Difficulty to maintain good pasture
Shoeing also disables the early detection of any damage done when the rider/owner is
exceeding the horse's biological limitations without being aware of it.
The horse does not show immediate symptoms until damage is advanced far enough to cause problems.
So, why would we want to shoe our horses?
Because it is traditionally/conventionally seen as correct and has become part of taking good care of the horse, forgetting its origin when shoeing was a indeed a "necessary evil" for a short period in the history of man and horse..
Any good farrier will agree however, that a healthy unshod hoof is a "better" hoof than a hoof with an iron rim attached with nails driven into it.

The romantic "clip-clop" of shod hooves and the way we think about horseshoes makes it seem a lesser "evil". But becoming more aware that the hoof of the horse is not just a hard structure at the end of the horse's leg, may give us an insight to help our horses to a healthier life.