"If I'm wrong then please somebody explain to me where is the role-playing part so I can appreciate it as it was mentionned on the box of the game"
Unless you're rich, you should not buy something based on what's on the box, or how the company that wants your money markets the game.
Do your own research bevor you buy a game. Youtube had gameplay videos from the beta since fall '08. You could have googled for "DotA", the word that is mentioned in more or less every preview of the game.
Finally, you can always simply resist to buy something so short after release (or worse: preorder). This is always a gamble, and it's never worth it. If you gamble you have to expect that you lose
"role playing" is a very general term. Mario Kart can be interpreted as a role playing game. The character building in Demigod is like that in Diablo, so marketing used the term here too.