Ok I give the whole DR thing another shot:
I repeat I'm DMin for 13 years (I play much longer but 13 years as a DM). I dont complain I usualy have good players.
Still there were alot of silly things after a while (usualy when party reaches 10th level).
It always struck me somwhere near the stomach when after describing the filthy demon pack that attacked the party everyone was just checking their inventory list to make sure they didnt forget to pack in the +3 weapons and checked their stats and HP and went like: ok were ready...bring them beasties on!
??? What a letdown...they knew from the beginning they would make it...why? Because they knew they were prepared, they knew they could beat them!
Thus reducing the whole MM just to comparing your stats to the Monster stat making sure you had that +3 weapon ready and possibly even starting to calculate exactly how much damage you will dish out per round and how long it will take you to beat it to mush.....wow what a challenge, how imaginative and fun, ...frankly those are the worst moments in the game.