"What do you think of that dress" is accepted. I'm surprised Duolingo is accepting any answer with "this" as a translation for "desse", being that they don't accept that slight change in most questions. Don't get me wrong, in speech we often use them interchangeably, but here they are usually more strict about it here. =)
Achar = to find, to think, to have an opinion Pensar = to think Honestly, in this context, the opinion of someone, they mean the same. But you can't extrapolate and say something like "os grandes achadores da Grécia" (the great thinkers of Greece), in this case, only the word "pensadores" is right. When you're talking about their ideas is ok to use "achar" - What did Plato think about love? / O que Platão achava do amor? / O que Platão pensava sobre o amor? When you mean "to find an object or a person", only use "achar".