Always state your reasons for leaving in positive terms - "I want a new challenge, I work hard at my job, and I'm looking for opportunities for advancement. I'd really like (this big advanced work that you're not currently doing but would like to do) and there's not much opportunity for that in my current role. this job is perfect because..." is great. "I'm overworked, underpaid, taking on more than my role dictates and never getting recognized in either money, promotions or interesting assignments" can come off like a rant.
Both are honest, but having goals and dreams is a very enticing quality in a new hire. Having a buildup of frustration and disgruntledness is not.
I would generically encapsulate your reasons as lacking vision of perspective in the current position and pursuing career growth. I am not sure about the culture in Italy, which is certainly a factor, but I can tell you that, in the U.S., honesty about hitting a dead end in your existing career or menial duties is in 98% of cases punished. Why? Because it is self-incriminating as an admission of professional shortfalls and you should never ever say anything in an interview that does not work in your favor of making a sale. An interviewer is not a shoulder to cry on, it is someone to make a sale to. I understand you may be tempted to be honest or to even rant about frustrations in your current job. Don't do it. Just like Chad said in his comment, those are canned questions and are not worth a well thought out and honest answer. So be pragmatic and don't forget the interview is not a shrink session, not a chat with your BFF, not a place to be overly honest -- it's the cusp of a market place and your mission is to make the sale.
There are a few pitfalls here I'd be careful to avoid:
Blaming the former workplace - If you talk down about where you worked before, then the people interviewing you may fear being in a similar situation in the future.
Follow-ups to these responses - As you give these answers, there may be questions from the interviewer of, "What did you do about these concerns?" that may not be a question you're prepared to answer in a positive light.
Putting yourself in a less than great light - This would be bethlakshmi's response where if you answer in less than positive terms, it may not go over well.
The Grass Isn't Always Greener
Do some research and decide why you think the new job is better than your current one. Just because one opportunity is better, doesn't mean the other one is bad. Although you may not have been compensated/recognized for your efforts, you current company does let you do more. You can now put these accomplishments on your resume/CV and get a better job.
Some interviewers may notice the discrepancy between your title and actual responsibilities. Let them know you took the initiative to do more, but the company didn't have the room to promote you. It happens.
Some day you're going to have to explain why you're leaving a good job.