The other magic items are cool too, but the effect of allowing them into play--at least for one-shots like the Undermountain mods at Gen-Con was that people would buy multiple copies of the efficient and useful items (like belts of healing for instance--and the outright broken items like belts of battle). If Undermountain had been higher level, I would fully expect to have seen characters carrying a half-dozen or more belts of battle and simply switching them out after each combat. That's not ending the Christmas tree effect--it's just changing the decoration.