I already have to watch Omen pretty closely as it is now, we have mages, DK's, warrs and especially warlocks atm doing insane amount of damage and therfore threat. There are more classes doing over 6k dps raid buffed but I only mentioned the ones that have the biggest trouble with dropping aggro with abbilites. The biggest issues imo are for Warlocks atm, at least in our raids, with only one abbility on a 5 min cooldown, it is a real problem. It may work now as most fights are ~3-4 minutes except the very end bosses in Naxx and Sarth/Maly. Ulduar will bring longer fights for a longer time span than Naxx did, what will we do then if our TPS is slaking behind and thouse classes can't maximize their DPS becasue we cannot hold our ground? It's simple: get a Pally/Warr or a DK to tank, they scale a lot better on threat.
It is not a problem if you are in a mediocre guild with average 3.5k dps-ers in their raid, indeed that is true. But there are plenty of good dps-ers and good guilds for this to be a problem, it's not just the top 100 guilds in the world that have shitloads of DPS in a raid. Each realm has at least 4-5 guilds compeating for server firsts and therefore they will have issues where bear tanks might see most of their raiding as kittys. I am of course waiting to see the final changes on the PTR, but I don't want us to have to wait for T8 and gear over 226 item level so that we see improvements.
Don't forget that a lot of classes are going to see a buff to their average DPS. Will we as bears scale in TPS in the same way they scale in threat? I can't asnwer that for sure as others surely tested more than I did on the PTR. Maybe more replies from top end guilds testing things like bear tanking can be added here. I am eager to hear more.