Which roles require sets and which require stats?
When we help people build their champs, we often say "stats over sets". But it's not quite as cut and dry, and gear requirements can be very important in deciding whom to build.
For example: an AOE nuker benefits greatly from Savage, Instinct, or Cruel sets. A dungeon solo champ is often required to have Lifesteal or Regeneration sets. On the other hand, a debuffer only needs stats (Accuracy, Speed), so sets like Perception and Accuracy help, but are not necessary.
What other examples of general roles/builds can you think of that employ specific features of sets beside stats?