Sure 1d12 crit was available, but not with other good effects on top.
For me the different weapons start to feel really different with the essential expertise feats. Its feats you want to get anyway, with every weapon, and makes them mechanically more different.
So for me having different effects on different weapon types does help.
Of course you can see some of these things being "flavour", but having the bludgeoning weapon having things like resounding weapon which can daze, and swords etc. not makes sense and does make them feel a bit different.
Duelists weapon plays into the "rogue" light weapon thing about wanting to have combat advantage, and its also something which is a lot easier in general to get on melee weapons (at least early in 4E), so it makes sense thats not also on range weapons (for balancing concerns).
Pact Blade is a weapon implement, and you dont necessarily want them on too strong weapons, so light blades only.
Berserker Weapon would be a lot stronger on a ranged weapon, because the bonus of being easier to hit will be a lot smaller disadvantage when you are not in melee range anyway.
So overall it is definitly a mix of flavour, making weapons more different (which partially is flavour) and some balance concerns.