Steel

It is the rare culture that develops steel.  It is first required that the culture develops metal manufacture early, in that it takes many, many generations to develop the intensive design elements of working with metal and creating alloys.  The culture demands an extensive fuel source.  The culture demands that there exists an abundance of food supply,, in order to support the existence of metalworking specialists who do not contribute directly to that food supply.  And finally it requires a large and varied cultural landscape, filled with war, that encourages the culture to develop methods of tool manufacture that would never be required in a peaceful, passive society.

The primary motivation for creating steel is that it is a far superior metal for use in war. 

Oh, obviously it has other uses which make it superior also ... but those are historically uses that were devised after the invention of steel.  An individual observed the capacity steel had in reference to a particular problem, and applied steel to that problem - railroad tracks, for instance.  Natural application resulting for observation.

Craving the hardness of something that did not yet exist, however, the discovery of steel was pressed for as an answer to swords and other weapons that were too brittle not to break, or too heavy to wield comfortably, or too soft to take an edge and keep it over a long battle.  Curiously, the culture that discovers the solution seems to be the culture most warlike, and most readily able to adapt the solution in terms of its cultural appreciation for human slaughter ... Europe, for instance.

But what of a culture that cannot manufacture steel?  To begin with, because it does not have enough skill, enough knowledge about the elements, enough resources ... and because it has no access to the creation of fire.

The first rule in the manufacture of metals is fire.  Fire requires wood.  There's always magical fire, but I have no examples of any sustained magic fire - metalworking requires days - and in any case, an interesting magical source would hardly be widespread throughout your campaign.

So how do orcs smelt metal weapons?  Obviously, they have iron ... presuming they'd bother hammering ore out of the cave walls when they have no means of burning a fire long enough to separate the metal from it's rock prison.  The same question comes to mind about a number of other metal-weapon using races listed throughout the game.  Where do these swords come from?  And why are they never constructed of bronze, so that they break the first time the party's steel swords connect with them?  Why is it that every culture, from every corner of the world, has the knowledge to make steel as well as every other culture?

Your world's culture and economy has an opportunity to be defined by the placement, abundance, invention and use of metal weapons.  A careful reading of Jared Diamond's Guns, Germs and Steel, mixed with some careful geographical distinctions of the parts of your world can create a "sword-running" climate that will provide years of meaningful outdoor adventures for your party.  It will establish dynasties and prejudices, define critical borderlands, give meaning to vast migrations, institutionalize slavery (and promote the need to abolish it) and so on.  The movement and manufacture of steel weapons can provide a solid ethical background to your world on a level you may never have considered.

What happens when one group can make weapons of the greatest destruction at will ... while another cannot beg, borrow or steal them?  Pun intended.  Consider how that is played out - and consider which side the players may be expected to take.  Is gunrunning acceptable?  Or is it better to support a culture free to slaughter its enemies at will?  Which is the redeeming point of view?

All from a simple thing like steel.  Your world is made right there.