hit and expertise are both more raid stats than anything else. hit is nice because missed attacks = less threat. for a 1h+shield user, you want about 8% hit total in order to completely eliminate misses while fighting a raid boss. you dont *need* 8% hit, and in fact its generally inadvisable to focus on getting hit as a tank except in certain circumstances, but having 3-5% hit isnt a bad idea.
expertise is even more important than hit, because it does two things. first, it eliminates dodges, which, again, increases your threat as well as your damage. secondly, and most importantly for a tank, it eliminates parries. parry is an especially important thing to eliminate (or in this case, reduce, since elimination of parries on a raid boss requires a LOT of expertise). the reason behind this is that parries not only block an attack done to a target, but they also increase the attack speed of the next attack to come. parries also "stack" so if a raid boss parries 3-4 attacks in quick succession, those 3-4 attacks will come back on the tank in short order. this is where a lot of the "tank got one shot" horror stories come from. generally a dps gets into a position they shouldnt be in and said dps gets parried 1-3 times and these stack on top of the bosses other attacks and instagib the tank.
for expertise, you want enough total to eliminate a dodge chance of 6.5% (this is about 22 expertise iirc). parries on raid bosses have a parsed rate of about 15% so the amount of expertise you would need to eliminate parries is therefore prohibitive. a good rule of thumb is to try to remove dodges from the table by getting 22 expertise and stop there. the exceptions to this are specific fights where high threat generation is more important than mitigation, at which point some tanks opt for more expertise in lieu of traditional defensive stats (never EVER go below the def cap tho, unless its on a fight that has specific mechanics that make being def capped a non-issue).