The Ferengi are considered ugly by almost all cultures in the Federation, but these traits give them many advantages.  Their oversized ears give them very acute hearing, and their four- lobed brains are non-psionic enough to give them psionic resistance.  This gives them protection against mental attacks, especially telepathic attacks.  This is a definate advantage for Ferengi, especially when cutting business deals with telepathic races such as Betazoids. No Ferengi can ever possess psionic abilities.  Though highly intelligent, there are almost no Ferengi scientists, and no formal schooling system in their culture.  Lessons on handling business deals are handed down from father to son.  Almost any technological advancement that Ferengi have benefitted from has been bought or traded for from other cultures. 

Ferengi have an odd honor system. They feel that cheating and stealing are acts of cunning, and that anyone who was a victim of their scams was not smart enough to protect themselves from them.  Generally known as crooks and robbers in the Alpha/Beta quadrants, they have looked toward opening trade in the Gamma quadrant via the Bajor wormhole. 

Also, the higher gravity of their homeworld affords them a higher base strength.  Even with this added strength, the Ferengi believe it is easier to run away from a fight than to risk injury.  A Ferengi who is or has been in the Ferengi military service will have a forehead tattoo.

Their mimicry abilities give them an uncanny ability to duplicate virtually anyone (or anything) they have ever seen.  They can change form, shape, size, and color at will, and can do so in just a few seconds.  Combine the above with their Body of Plasm, and they are a very effective double agent, among other things. You would have to know the victim of a Changeling switch -very- well in order to detect their impersonation at all. 

The races of the Alpha Quadrant have recently discovered that pieces removed from a Changeling's body will revert to their natural gel-like form. Blood tests are now standard protocol throughout the Federation and Klingon Empire to weed out Changeling infiltrators.  Members of this race can stretch their bodies even while maintaining a regular form. They do not need to eat or drink, at least not in any known manner, and their need of sleep is quite different from normal humanoids. They do not even need to breathe, but their bodies cannot survive in the vacuum of space without a suit of some type. 

Changelings have a fanatical hatred of  "solids" and have attempted to infiltrate militaries and governments of several Alpha Quadrant races, including the Federation.  It is not known how far this process has gone.