This isn't DnD. It is Diablo and it has its own mythology. Paladins were knights, pure warriors meant to protect the missionaries, that impressed the heathen more than priests did and eventually replaced them, while remaining 100% warriors. And besides paladin casters (hammerdins etc) there are barb casters as well, the bards/singers.
Paladin is the Iron Clad knight with his hard to pierce plate armor. Makes perfect sense to me that the man behind all that heavy metal and 5 foot shield is the hardest to get hit. A paladin gets no life bonus (bo/shifting/oak),no minion stack,no d/a/e. He has only this, his awesome defense and block, both tied to his shield. His class specific item is the shield, the ultimate defensive item. Barb is the offensive dual wielding warrior and pally is the defensive/shield using one, simple as that. I'm glad that both in pvm and pvp, the most potent barb is a shieldless dual wielder (frenzy/bvc respectively), blizzard got that perfectly right. Barb is all about offense.
Druids are supposed to cast in human form and melee while shifted. Bear has the defense,wolf the fhr and both good life. The fact that they are indeed overlooked by blizz doesn't mean that the rest of the game should get messed up. I do support taking the bear in pally/barb defense levels, albeit it seems to me that shifters resemble barbs more, having the life and/or the ability to recover quickly after getting hit. In any case, shifters suffer more in the offensive area, especially maulers. It is a pity that there are literally only a handfull of weapons for them.