Sets:
These have been around for a long time in D&D, but unlike Diablo, these "sets" were always of artifacts and near-artifacts, not items that you just find lying around any random dungeon.
Crown, Orb, and Scepter of Power, each set dedicated to Good, Evil, or Neutrality, meant to be the tools of the Champion of that alignment.
Teeth of Dahlver-Nar.
Eye and Hand of Vecna.
Cup and Talisman of Al'Akbar.
Even the Girdle, Gauntlets, Hammer combination is meant to echo a much more powerful set of artifacts: the Girdle, Gauntlets, and Hammer that Thor wields.
So, the "sets" in Magic Item Compendium had better be of much weaker power than the above, or they're not going to be showing up in my games at all.