Akira Toriyama's sin was to not giving much attention to this awesome character.
Why he is awesome:
1. he is the only Dragon Ball Goku's friend who was able ti defeat him once. And the only one in the 23 Tournament who was strong enaugh to give to Goku a good fight. 2. his ability to learn any skill pretty fast just watching. 3. He always was brave, he never step back even when the enemy was a lot more stronger than him (Nappa, Android 17, Cell, Buu) 4. Tenshinhan is maybe the Dragon Ball Character with the biggest arsenal of martial arts tecniques, he is like a living encyclopedia, only Cell knows more but everybody knows why.
Im think the reason why Akira didn't him a better attention was becasue there was too much characters in the serie and he was too much focus in characters who can transfrorm and stuff like that.