motive has shared some details on the calculations of hit and crit chances:

part 1

The way WoW calculates crit rate is over ALL attacks. Crit rate is not based on hits only. In other words, if you have a 5% crit rate, that 5% chance includes misses.

All crit rate adjusting abilities, items, and talents add the flat % to the base % crit rate. So if I have a 5% base crit rate and then use an item or talent that increases that crit rate (let's use Improved Backstab talent for example - +30% crit), my new crit with backstab is 35%.

Regarding how defense decreases the rate of critical strikes, each point of defense that a target has over the attacker, the attacker loses 0.04% chance to crit. So, for example, if a level 60 Rogue is attacking a level 60 Warrior who has 25 defense, the rogue's crit rate will be decreased by 1%.

part 2

+toHit items subtract from your miss%.

So, ignoring all defensive actions (Block/Parry/Dodge/etc..) if I have 20% crit chance, 20% miss chance, and 60% hit chance and I equip an item that gives me +5% toHit and +5% crit, my stats become 25% crit, 15% miss chance, 60% hit chance.

New hit chance = (Original hit%) + (toHit modifiers) - (crit modifiers)
60% + 5% - 5% = 60%

New crit chance = (Original crit%) + (crit modifiers)
20% + 5% = 25%

New miss chance - (Original miss%) - (toHit modifiers)
20% - 5% = 15%