Very interesting discussion indeed.

IMHO, there are 3 levels of language difficulty.

First, all languages are difficult, since they have to be complex in order to reflect the complexity of the real world. If they were not such complex, they would be completely useless, as we wouldn't be able to express anything.

Second, what actually matters for difficulty of studying a language is a level of similarity between languages. It's obvious a native English speaker could relatively easily learn French and would have some troubles with Serbian, for example.

And third, in addition to "default complexity" each language has its own difficulties. I mean such things as irregular verbs, unpredictable usage of some prepositions and particles and so on.
For example, when we want to express something is placed somewhere, we use prepositions "in", "on", "at" in English and "в", "на", "у" in Russian. In both languages, usage of these prepositions cannot be fully explained logically. Often we just have to remember what preposition should be used with a specific word. В школе, на фабрике, в парке, на улице, в театре, на выставке etc.
Other examples are grammatical gender, different classes of verb conjugation and so on. Табуретка is feminine and стул is masculine for no reason, that cannot be explained in any way.

A typical language can have dozens (or even hundreds) of such irregularities. I think, they are the most annoying thing when studying a language. When one says English is difficult because of all those terrible 12 grammatical tenses, I can hardly agree with that. English tense system can be explained with 3 or 4 simple rules, and these rules have almost no exceptions. But if there were a rule that a half of English verbs should never be used in Progressive Aspect but only in Simple, and there were no way to guess what these verbs are, that would be really annoying and difficult to remember.

English is a highly regular language. It is huge, complex, contains millions of words, but it is not overcomplicated. I think we were lucky to have English as an international language. If it were Latin instead... Б-р-р-р... that is scary!

So I believe if there are a way to objectively measure language difficulty, that way is by comparison of irregularities and exceptions between different languages. From that point of view, such languages as Latin, Russian, Lithuanian and Greek are much more difficult than English, Turkish or Quechuan.