In the Saiyan Saga (don't recall which episode, but I believe it was when he was with Nappa on another planet), Vegeta mentions that Saiyans who were born from humans and Saiyans are more powerful than pureborn Saiyans. This makes sense when you see moments of Gohan surpassing Goku in power level when he gets angry, such as when he fights with Garlic Jr. or Raditz.
Later, we see him being a teenager and surpassing Goku definitely by becoming SSJ2 first, but we don't see anything of this sort with Trunks. Trunks apparently surpasses Vegeta in a fight with Cell, but then we know Vegeta wasn't using the buff form because it makes you slow. And Gohan surpasses Trunks even more when he's 10 and Trunks is chronologically 19 or more (2 years in the time chamber). Later, Gohan doesn't train and Goku surpasses him, but then again when his hidden power is released he becomes the strongest character for a while again.
Gohan doesn't train much, but if he does he becomes the strongest. Why doesn't Trunks have this "hidden" power, or is able to surpass Vegeta when he gets enraged or after a hard training, as Gohan does with Goku? Trunks is also a descendant of a human and a Saiyan, so shouldn't he be much more powerful than what he is?
Edit: Now things have changed with the new Dragon Ball Super episodes where
Trunks can surpass Gohan, Vegeta and Goku in power when he gets angry, and it seems to have as much hidden power as Gohan. He also, when enraged, gets a new transformation no other Saiyan has obtained before.