Well, the important thing is, from the sound of it, everyone is enjoying the game.
I know that to challenge the characters, for the big guys anyway, I'd probably have a spellcaster who isolated some of the characters with walls of force or stone. It makes healing someone a little more difficult.
In materia Magica, there's this one magic armor property/ability called reboudning. A paladin in my campaign has a suit of leather armor +4 with it. 50% of the time it bouces half of the damage unto the enemy. He's taken out many a foe with it. Curse Improved Toughness and his horde of hit points!