To start off: I am not going to worry about dev intention or not. They can decide if they want ret to be able to tank. This is purely about its ability to tank.
This is mostly based off my experience a few minutes ago tanking VoA as ret.
I'm not too concerned about aggro. Ret is definitely lower than prot. A lot lower. But, I think it's high enough. People might have to hold back at high gear levels, but it's not as if they're reduced to wanding.
Still, scaling is a concern. So much damage is based on crits and tanking gear lacks crit. In addition, SoV doesn't crit (well okay, the instant 4 damage can crit), so all the crit multipliers have a reduced effect. With the change of DS to physical, ret lost a big piece of holy damage and at this point is going to be doing quite a lot of physical, so the RF multiplier is weakened. Fortunately both specs get divine strength, so that scaling at least remains intact.
Adding RV to CS may help, increasing the chances that the holy DoT will stay rolling.
Ret takes more damage. It's a lot more. This is completely okay, after all, prot is the tanking tree. However I think ret in proper tanking gear can survive in easier content (such as VoA) or when overgeared (I was also overgeared).
Ret can pretty much use a modified 6-9 rotation. CS, judge, SotR, consecrate, CS, DS, SotR... It's not quite 6-9 since DS is 10, but with lag and slow reactions it fits.
This is a problem. I was feeling a noticeable shortage of mana even with DP. JotW wasn't enough. SA wasn't enough. I was losing mana pretty fast where as prot I'd stay about even or slowly fall. I think the biggest thing is the lack of BoS. It will get much worse when SA becomes a talent. Will buffed JotW offset this? I doubt it.
Overall I think ret is able to offtank if geared properly, but with 3.1 and SA moving to a talent I see potential trouble coming up.