Barbarian is definitely better than Knight, Knights have basically no purpose except in terms of ease of equipping (if you find Obsidian Plate Armor before you have Obsidian armor to equip on everyone else then you'll be able to stick it on the Knight).
I'd similarly say that Ninja is almost strictly superior to Robber. Thievery levels up anyway. If you pick a race that receives a thievery bonus as a Ninja you'll be equal to a standard Robber anyway. Ninjas are just as good as Knights ignoring minor equipment differences.
Full caster vs. half caster is kind of funny. On one hand, half casters are still basically just as shit as full casters at actually hitting things with other things. On the other hand, if you are properly (ab-)using your abilities you can warp back to recharge at almost any time, negating the only real penalty for half casters (half magic pool), and the extra ease of finding good equipment with classes that can take more stuff is useful. If you run something like 2 Barb+Ninja+3 full casters you'll definitely hurt throwing away tons of good equipment early on until you hit the point where you're drowning in Obsidian for everyone.
FWIW, Levels per attack by class: Barbarian 4, Paladin/Ninja 5, Robber 6, Half casters 7, Full casters 8. So you can see why going Paladin->Barbarian or Robber->Ninja adds a lot more damage per turn than Full caster->Half caster. EDIT: Actually this is wrong, only sorcerers have 8, other full casters have 7 with their half-counterpart having 6.
Agree pretty much on nature-type spells being not useful in WoX, but they are additionally not really useful in 3 since you can just hire the druid (or ranger?) who comes with the important spells when you need to water walk. But then 3 dropped even more loot on you to the point where classes really didn't matter anyway. Like, ideal party for 3 is 6 barbs and you just abuse all of the enchanted items to cast whatever spells you need.