If it were move equivelant actions to activate the properties, I wouldn't have any problem with it. But say you are wandering through the woods when the ranger spots Ogre-barbarian bandits ready to ambush you. He calls out and the encounter distance begins at, say 100'. The fighter draws his sword as he moves 30' (Thankfully he didn't get surprised by the ambush). Now, the barbarians do a double move, getting past him, with one stopping to deal with the 'ittle 'uman. The fighter activates his elemental property and then ... oops, can't attack. Next round, he can choose to attack, or try to get value out of that second elemental enchantment.
It does provide a disincentive to use stacking elemental echantments. But, it isn't quite the same as the wizard with the staff. Each time the wizard uses the staff to activate a power, the affect is noticable immediately. Even at 20th level, a flaming sword will add at most 5d6 damage a round. Which is the same as the entry level for fireball. Yes, a saving throw is unnecessary, but fire resistance still works. So, it doesn't seem the same as a multi-trigger spell staff to me.
I don't see where it is abusive in damage, nor where it really invalidates the abilities of another class. With the new DR changes, yes it become less important to distinguish between the +1 sword and the +3 sword, and the extra +2d6 elemental damage becomes very appealing if there is no cost associated with it. But, is that abusive enough to make it difficult to always get that benefit?
That's why I ask really. I _know_ that if it is terribly abusive, somebody here can show me. Of course, if it isn't terribly abusive, I still waffle on it. Maybe that's the answer I need. If it's abusive, I can see an easy 'No', but if it isn't, I am still not sure on a 'Yes' to activating everything with one command word. Hmm.