Well, there are many reasons why Polish is so hard to learn. For me personally, the most difficult are cases (yes, there are rules, but usually cases have more endings, and you simply have to know (so to learn) why that ending is used and not another, for example Genitive of masculine inanimate have 2 endings: U and A: i go to kina, sklepu, basenu... Polish people simply know/feel when to use which one, but foreigners like me have to learn it :-/
Another thing is conjugation of numbers. Common, why to make life so complicated? :D
What also causes me lot of problems is pronunciation. So many consonants in a row, difficult to get used to it.
People say the more language you know, the easier is to learn one more: unfortunately it doesn't work with Polish. I do speak some languages and still have problems with PL :(