This is one of those post that you are going to get a lot of opinions. What I always tell people is that you want to keep lvl stats in all your gear. Don't give up a great piece because your hit or expertise takes a hit (we are tanks not dps). Blizzard has work diligently to move each tanking class towards blizzard's design with gear. Take upgrades and adjust. Stay defense capped (540 - for those who missed it) and play around with Hit and Expertise in slots where maybe you can catch a socket bonus. Look at it this way --- getting parried add expertise; missing attacks add hit rating.
It will vary if you're talking single target or multiple target. Expertise until the soft cap I think is #1. maxdps.com actually has tank gear rankings now they have exact numbers just move the slider over to threat and plug in your stats. Unfortunately it doesn't consider staying defense capped so it recommends dps gear - but it does give you dps #s for each stat. I wouldn't take it as the bible truth but if you're too lazy to investigate further it's a good guideline. I haven't found a lot of tanking pieces that are so close that I need to do the math yet but if I were gemming solely for threat I would probably gem strength - even if I'm not expertise capped because expertise doesn't buff your thunderclap, shovewave, or damage shield and threat wise our biggest weakness is AoE threat. I haven't found any dps that can do over 6000tps and if I did I'd be so very happy.
From the first week of tanking both in 10 and 25 man Naxx, i've found having low expertise really kills your threat generation if you're constantly seeing Parry on your screen, as i was. I've seen 6k from DPS on Loetheb but luckily thats one fight you don't need to care about threat I dropped some crit from my build but now i've seen our DPS putting out 4k+ in 10 man Naxx and i'm starting to wonder if losing threat now is a result of this.