Fury and Arms give up considerable damage while they're sitting in D-Stance, not generating rage from normal attacks. That doesn't compare to free passive gains from all the boosts Prot Warriors enjoy which would need to be addressed (not least of all armour bonuses - on my char it's 45% as Arms, 61% as Prot). Then you have the versatility of Barrier (and Block) for fast mitigation. I think my Warrior's Barrier with 0 Vengeance (or buffs) absorbs roughly 100k. To mitigate that much with D-Stance would mean taking something that would've hit for 400k damage whilst in it.
The armour is a big deal on any boss that deals raid wide physical damage or uses physical abilities. They do exist and it's something you'd have to consider, especially when they're looking to make armour a valuable enough stat to appear on gear.
This is why I'm not sure about it even though I want it. They'll have to do quite a bit of jiggling about to make sure Gladiator Warriors aren't too good in PvP or too good at DPS with only advantages over the other 2 specs or that they aren't so bad at DPS still that nobody will ever touch that talent. I can honestly see it going the way of Demon Hunting where they just decide it's too much effort and change it. It just seems way too ambitious for a single talent, when big playstyle changes should really have a whole tree behind them. Which would be a shame because it's the thing I'm most interested in trying out.