rules yes..but game mechanics? can any other army run and shoot? or embark in combat?
Tyranids and Space Marines basically ignore the way morale and rallying work respectively (and Daemons pretty much exchange it for a whole other system). Space Marines and their ilk ignore the way units bought for the army work, with Combat Squads, and are basically the only army (along with GK, who are close enough to be the same thing) that can bring Reserves in on the first turn. Necrons ignore the rule that you may only use your best save (technically, anyone can
do this with FnP, but FnP is easily negated in many cases and no other army can bring it, unconditionally, on every model for every turn of the game). Imperial Guard ignore the way the FOC works with almost everything in their army coming in as a Platoon or at least a squadron. Dark Eldar have a lot of fixed To Wound rolls that ignore the S/T comparison chart mechanic.
Harlequins do horrible, horribly leadership shenanigans where they can kill a guy and get his squad to run off an objective, or cause horrendous amounts of damage by deep striking in a Raider with a Webway Portal, the Mask of Secrets and the Armour of Misery or getting some Horrify from Warlocks and then casting Laugh of Sorrows, Terrify, Psychic Shriek etc., but they don't really stand up as their own army being a very fragile, very expensive melee force. Can make a great detachment to add to a DE or Eldar force, but not to be considered viable alone, I think.