Interesting, until defensive ability warrior was pretty much relegated to bench warmer in my recollection.
I agree though, if among themselves the defensive tanks are going to wear the same armor and have roughly the same hps but the other two have nifty spells, then warriors need something either similar or some difference to make them just as "good". It seems to me an edge in defense or hitpoints is not where the answer is.
I'll defer to the more experienced warriors here. I just started mine and he's only level 21, so my experience with the class is still very fresh. My mind, thinking this over though, says that mitigations and hps are less important when compared to the ability to hold agro. A defensive tank with all the hps and mitigation in the world is nothing if they're not using it. This isn't a comment about warrior's current ability to hold agro but just an observeration, personally on my shaman I love grouping with warriors (and paladins) because they seem to hold agro pretty good.