Enchantments (at least from the enchantment table) are not weighted, causing all enchantments to be equally likely. This makes it needlessly difficulty to enchant items, particularly armor and swords.

Protection should have weight 10, Fire/Proj Prot weight 5, and Blast Prot weight 2, meaning you should have a 45% chance of getting protection over a different protection, but instead you have a 25% chance. On the other hand, thorns is supposed to be one of the hardest enchantments of all to obtain with a mere 2.9% chance per enchantment, but instead it has an 11% chance (and in fact is more likely than even protection as if proj/fire/blast prot is chosen as the first enchantment, protection has a 0% chance while thorns is 20% for any additional enchantments).

This affects all enchantable items.

Steps to reproduce:

1. Set up a full enchanting table setup in creative.
2. Enchant a large amount of bows* at level 30 (a double chest should suffice).
3. Tally up how many of each enchantment there is.
   The five enchantments occur at roughly the same rate.
   Expectation: There should be roughly twice as much flame and punch as infinity (each), 5 times as much unbreaking as infinity, and 10 times as much power as infinity.

* This applies to all enchantable items, but it is easiest to demonstrate with bows as they have no mutually exclusive enchantments, and their weights are very diverse.