Name and Location
Each Super Unique enemy has its own name, never used again anywhere else in the game, and is always found in the same location. For example, Blood Raven will always be found in Act 1, within the Burial Grounds.
Abilities and Immunities
Each Super Unique enemy always spawns with the same ability. On harder difficulties, they will spawn with one or two additional abilities, but their first ability will always remain the same. For example, Blood Raven will always spawn with the Fire Arrow ability. Certain Super Uniques may have complete or partial immunity against certain spells and skills, such as Crushing Blow or stuns.
Considerations
Because of their fixed location, Super Uniques often make a good target for characters who aim to kill as many named enemies as possible in the hopes of obtaining higher-quality items. For example, the Countess, found in the Forgotten Tower, has a high chance of dropping one or more runes when killed, while Pindleskin, found within Nihlathak's Temple, can be reached very easily and has the potential to drop the highest-quality items in the game.
Special Super Uniques
In addition to normal Super Uniques, "Diablo 2" also has several special Super Unique monsters that can only be reached and fought when certain conditions are met.
Once your character has killed Baal in Normal, Nightmare or Hell difficulty, you can open a portal to the Secret Cow Level in the same difficulty; the Cow King, a Super Unique that can drop many rare items including an exclusive set, can be found within.
Players selling many Stone of Jordan rings on the same server will cause Uber Diablo to appear in place of a normal Super Unique in every game on that server; Uber Diablo drops a powerful unique small charm, called Annihilus.
Collecting three Keys of Terror, three Keys of Hate and three Keys of Destruction, rare items dropped by the Countess, Nihlathak and the Summoner in Hell difficulty, allows you to open special portals to the Matron's Den, the Forgotten Sands and the Furnace of Pain. Collecting an item from each of the bosses found within those locations will allow you to open a portal to the Uber Tristram area, where Uber Mephisto, Uber Baal and Pandemonium Diablo can be found. The last of these Super Uniques you kill will drop a powerful unique small charm called Hellfire Torch.