Robert Schwalb talks about the dated "+1 or better to hit" mechanic. This was something I saw in 2nd Edition, might have been in 1st Edition, existed kind of in 3rd Edition, and surprise sur-fucking-prise...I hated it.
On one hand, having a monster that can only be harmed by magic sounds like it could make for some interesting challenges. If the fighter lacks a magic sword, or his plus is not big enough, then it is up to the wizard or cleric to save the day (which given how fighters do not scale sounds less interesting and more like foreshadowing)! On the other hand having a monster that can only be harmed by magic sounds really fucking frustrating.
The thing is, some monsters in older editions used to be immune to all damage from non-magical weapons, meaning that you needed a +1 or better weapon to hurt them. What if you group does not have a wizard or cleric? What if the wizard or cleric died or ran out of spells on the way? What if the cleric or wizard used up their offensive spells, or even the utilitarian ones that could just lock them down via save-or-screw? What was even lamer was that if the monster needed a +2, but you only had a +1? Fuck you. No benefit, period.
3rd Edition made things a bit easier by making it so that a fighter could hammer away at a golem, but you have to shave away damage from each attack to the tune of like, 20+ points of damage. The problem was that, oh yeah, fighters do not scale, so that basically meant that you were not going to do shit. Yeah, the wizard or cleric could buff you (and to be fair against golems they basically had to given the magic immunity), but in cases not involving golems it was probably better to just attack it with spells that A) were not affected by resistances, and B) scaled.
Revised Edition made things much, much better. Fey required cold iron to overcome, lycanthropes required silver, skeletons took full damage only from blunt weapons, zombies from slashing, and all the plusses were simply rolled into "magic". Oh yeah, and the amount was dropped into the 5-30 range (as opposed to topping out around 50 or more). So it sucked for the melee classes, who were already floundering in it, but did not render them obsolete.
That is basically how I would like it: make it have an impact, but not so much impact that classes have to pack it in and go home. Earth elementals I guess could be resistant to non-magical weapons, but do not make it so that the fighters just shrug and watch the spellcasters win the game. On a similar note, I don't want to see monsters outright immune to magic, or even item-eating monsters. I guess, ultimately, one could include both systems. Use damage resistance with an option for a DM to flip it to damage immunity or just ignore it.