Traps/hazards are treated like monsters. If you have a book of monsters, it makes sense to have a book of traps. I routinely skim Dungeon adventures for new traps/hazards/terrain. There is also a constant request for more traps in the Houserules board, and on the WotC board.[/list]
Do these hit the spot for others?
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The others seem to me more likely to have wide appeal in the context of a scenario. Is 4E really so rules-heavy as to make a rules-set-specific book of traps a good seller? How much really is there to say about generic terrain and hazards?
I personally think terrain is something that is missing. Interesting terrain is something that the DM can forget about when designing an encounter. I particularly love all the terrain in the DMG (The mirror crystals? HELL YEAH). Seeing a whole book of them to pluck ideas from, damn straight. In fact, a book of terrain is my first suggestion on page 1 of this thread.
The issue is not "It's rules heavy". The issue is "Here's an idea you can drop into an encounter to mix it up".