First of all, it's cool that you're asking. Most people have no idea what the definition of addictive personality is. Then they automatically decide they do NOT have it and use this as an excuse to go out and get high everyday saying things like "it's cool, I don't have an addictive personality, I'm not going to get hooked!" right out of their ass.
An addictive personality is a compulsive disorder. Although it applies directly to drug addiction, it can also be applied to eating disorders such as anorexia and bulemia and it can also apply to people with a gambling addiction.
I can give you a quick example of what an addictive personality is like. Back a few years ago, my friends and I would often party together. Do the same drugs, do the same amount of drugs. But that was it for them. They could party on the weekend and then go back and resume their normal lives. For me, drugs were my life. Drugs weren't just something fun I did in addition to work and school. I put drugs ahead of everything else and used them all the time and felt a need to be high.
As said above, there are different degrees of an addictive personality. Some people are worse than others. And until you've really become a full-blown drug addict, it's somewhat difficult to really gauge. Most drug users go through some period of heavy regular drug use.. what can make or break you is if you're the person trying things out for a month or two or three and can get over it and end the experimentation, or will the use just never seem to end and go on for months and years?
Also, the drugs used do not matter. First, I've got to say that the idea of seperating hard drugs versus soft drugs is ridiculous. Because if you have an addictive personality, it does not matter. When you have an addictive personality, you are hooked on getting high before you are hooked on the drug itself. Someone with an addictive personality can be entirely dependant on marijuana and smoke it several times a day every day for years.