1) Fluff - mainly the ex-supreme race/verge of extinction/tech humans can't comprehend etc
2) Models/style - original metal Scorpions and Banshees are what brought me into the game
3) Overall look - the only army where every unit being a completely different combo of mixed, bright colours works absolutely fine
4) Not a main thing but the 'Path of' series of Black Library books really got me back into the Eldar mood swing of things