The problems with traps are: weak damage, no effect on the many flying critters, and it drops at your feet, so they aren't convenient to place. It's so much easier to just outright kill a monster with direct damage.
I think traps are rare enough to justify making them very useful without the players able to "spam" them. It's be nice for players to be happy to find these when they get some.
If you want to keep them as "bear traps", I like the "freeze enemies in place" idea, but with only affecting non-flyers, and needing to place them in the path of enemies, it would have to be for a fair bit more than a few seconds to be worthwhile for me, enough time to actually get something done like separating out a clumped group of critters on an anachronism mission.
- Remove all damage from traps
- freeze enemy in place for 15 + <random extra time> seconds
- enemies can still shoot during this time.
- bosses are immune
- traps "tossed" toward cursor, instead of dropped at player location
Alternately, what if we made the bear traps into "proximity mines":
- When a creature comes within detection range it will trigger an area-effect blast over 1.5x that range. (Any less, and it would likely only catch the triggering creature, making the "area effect" pointless)
- The detection range would be large enough to trigger on a flying enemy passing overhead in a typical hallway room.
- The area effect damage affects all enemies in its radius, and is on par with a Fireball of the current world tier. (This isn't that much damage, considering the player's actual fireball would be boosted by enchants and likely be just one of many fireballs shot.)
- mines are placed like the Light spell, sticking in position.