I'm playing an invoker and was looking for some of those interesting conditional damage options. I combined Gauntlets of Blood with Bloodthirst, figuring with multi-target attacks I could often find a bloodied target. In the last 4 encounters we had, that extra +4 damage came into play twice I think, and I am always on the lookout for bloodied targets so I can use that little bonus. but bloodied targets die so fast, I rarely get a shot at them (and about a third of the time when I do, it misses). So a feat and an item slot yields +4 damage every other encounter, that seems very poor. Perhaps tactically I could just hold my actions until something got bloodied, but that doesn't seem very economical.
Bonuses on opportunity attacks, or combat challenge attacks, or when you are critted, are not very controllable conditions. So even if an item gave me +10 damage on opportunity attacks, I'm not sure if it would be enough of an attraction.
Things that can be tactically attained, such as flanking, attacking with combat advantage, attacking a marked target, attacking a target with no adjacent allies, etc, are the better conditions for such bonuses, and I'd like to see more bonuses for these controllable conditions.