It's quite simple, Ripple takes the brilliant idea of bitcoin and removes the brilliant part while calling it something new.
Instead of keeping it decentralized, to avoid one group taking control and dictating to everyone else, it centralizes the whole process for the advantage of the few(Ripple's devs and it's financial backers). This is 100% against what Bitcoin is about, and one of the many reasons why so many dislike Ripple.
What makes it a "scam", is that it claims to want to coexist with Bitcoin while at the same time working towards replacing it. You said it yourself in a reply earlier
You're not "replacing bitcoin", you're only replacing a large portion of what makes Bitcoin valuable(its ability to be used as a medium of exchange). If you can't see how that IS effectively trying to replace Bitcoin then I don't know how Im going to convince you.
I thought that bitcoin was just about double spending prevention cost erasing.... Ripple do that too isn't ? I'm not sure that the "bictoin idea" is about being decentralized....I think it's about improving payment system resolving smartly and costless the double spending problem...