From Diablo Wiki
Runewords were a feature added to Diablo II in the expansion pack, which used a combination of runes in socketed items to "craft" a completely new set of stats for it. This feature will not return in Diablo III, though there will be many other types of item crafting and creation.
Removed from Diablo 3
The D3 Team's rationale for not bringing runewords back is based on several factors. The designers thought that RWs were overpowered and led to a lack of equipment variety, but more fundamentally, they think they've got better ideas for items in Diablo 3. Better item variety, and better forms of item crafting. Here's Jay Wilson speaking in December 2008.
- ...we took a few things out, like Rune Words, essentially because Rune Words is a very simple crafting system, and we're planning to do something different there.
Diablo II Runewords
- See a full listing of in the Diablo II wing of the DiabloWiki.