The difference between the Tank classes

Well:
Arms has to double spec (pole/2handed AND a dmg type (thrust/slash/crush)). Most common would be 50 pole (anytime snare and defenders rage included), 42 shield, skill + rr + SC in either weapon line (all three have their uses), left overs in parry (xbow is not worth speccing).
This would leave you (when tanking) with 3x in a weapon line, 42 shield and a low parry spec.

Hero does not have to double spec and can go 50 celtic spear/lg. weapon, 50 shield, 2x weapon line, rest parry.
This would leave you (when tanking) with 2x in a weapon line, 50 shield and less parry (going 42 shield, more weapon and/or more parry would be an option tho).

Warrior does not have to double spec and gets 1 and 2handed allike by going 50 weapon. He can then either go 50 or 42 shield and the left overs in parry.
This would leave you (when tanking) with 50 weapon, 42/50 shield and 41/31 parry. 42+41 has a higher defense but 50/31 is better when blocking for teammates.

TLDR:
Warrior can both use 2handed weapons AND has high shield/parry; he also is on a higher weapon skill table than the other two. Hero however has all 3 dmg types and a higher weapon spec than arms (plus his moose which is basically a ghetto-IP) while the arms has the pole styles and a little higher absorb than the other two.
Alb and hib can pick saracen/lurikeen with have higher dex and/or qui than the mid races resulting in a minor boost to block- and parry chances. Trolls, half ogres and firbolgs are somewhat similar in str, con and their rather low qui values.

Feel free to add to this...