Swords.
Specifically, everyone having swords.
Here's the problem with it: as has already been pointed out the majority of modern fantasy takes it's cues from Tolkien. The thing is, Tolkiens setting isn't the high-medieval world most people imagine it to be but instead is much more focussed in a Dark-Age setting. Seriously, reread the battle scenes and you see alot more about shield-walls and byrnies of mail then you do knights in shining armour.
Now, in a Dark-Age (or even most Medieval periods) setting making a sword that doesn't snap the minute you hit something with it is really extremely hard! Not to mention that the amount of metal, work time and effort that goes into it would be beyond most people, even beyond many Smiths.
You know what are super easy to make and use very cheap materials? Axes, spears, clubs, knives. In fact, spears should be the most common weapon in any fantasy setting because you can use them to hunt your dinner with when you're done stabbing an orc.
Live in a wooded area? You should probably have an axe too!
All a sword does is kill people and no random peasant or minor lord is going to waste money on something that useless, only a King or Great Lord would have a retinue of men around him who literally exist to kill things and so get swords.