The Crit Cap and You
Once you start collecting a lot of very good gear, especially Icecrown-quality gear, you will start to bump up against the crit cap. Due to the way the combat table works for white attacks, if your chance to glance, miss, be dodged, and crit all add up to 100%, you cannot increase your white crit rate any further.
For ease of use by average players, all final crit cap numbers are as they would appear on your character sheet in a standard 25-man raid, and assume the presence of all raid buffs. This number will be higher than your actual crit rate on the boss, due to the level difference. You should check it in combat, with permanent effects (including your idol) active.
To find your white crit cap, subtract your chance to miss (8% minus your hit chance) and your chance to be dodged (6.5% minus your expertise's dodge reduction) from 77.8%. For instance, with no hit or expertise rating at all, but specced for primal precision, it's 77.8% - 8% - 4% = 65.8%. If your crit chance was higher than that, your white attacks would not crit more, however your special attacks, which are calculated differently, would.
Once you hit the crit cap, models suggest you should in fact stack hit and expertise to their respective caps. This moves your crit cap up, making crit more valuable again. Once you reach the hard crit cap of 77.8% (again, character sheet number), the hit cap, and the expertise cap, you should gem armor penetration to cap if you haven't already (soft cap if you have a trinket that grants ArP, hard cap otherwise), and then finally, most likely strength beyond that point.
A recent hotfix to boss crit suppression removed a 4.8% chance to hit that couldn't be overcome. If you see in another guide either 76% or 71.2%, it's outdated.
Since this guide's aim is for you to have to do as little math as possible, I give the number that you will get if you look on your character sheet while in combat. Assuming hit and expertise caps, you will become white crit capped when you have 77.8% crit on your character sheet.