Aggression, color, personality, the way they interact with each other, the fact that there's so many variants, you can even find two individuals within the same species that behave and look very different. Also the fact that they are hard to keep, it's a challenge to create a successful cichlid setup and you can never get bored because there's always a new mix you can try, new species, different biotopes, etc, etc. You might have a working setup and one day you wake up to find one of the lil ****ers has snapped and killed their tankmates lol... it's like a big gamble and that is just fun.
Plus they won't take a dump on your carpet or eat your gfs shoes...