Casual content is not difficult enough that any class will underperform to the point of being left out. Paladin's problems in Savage Alexander 3/4 have basically been in the making since 2.1ish, but for specifics:

-Weaker MT DPS than other tanks
-Their Utility/Support abilities being too weak in general (boss immunities, long cooldowns, Cover's general mediocrity, etc)
-Dark Knight's strengths being played to
-Paladin's strengths being largely negated
-Importance of INT debuff when it only comes from DRK and MNK
-DRK/PLD being an invalid tanking setup
-Other tanks have the same toughness

If Dark Knight's strengths weren't played to as much you'd probably just see a lot of warrior/warrior. Paladin's defensive advantage (as small as it actually is) doesn't mean diddly if other tanks can survive all the same stuff. Paladin's support/utility also is more plentiful but less useful than the other two tanks' (Slashing debuff/Damage debuff vs Reprisal/Int Debuff vs Cover/Veil/STR debuff/Heals).

Anyway, this stuff is pretty much why I switched at A3S. I really didn't want to. Paladin's skill set in general lacks synergy that WAR has (and to a lesser extent DRK). It didn't matter as much in 2.0, but it's starting to show a lot now.