A thought I had recently was how learning a third (or further) language is different from learning a second. With the second, you can still bend the rules in your mind from your native language to the second language, thinking of all differences as exceptions. But the third (and further) languages, you realize that languages just vary, and although comparing to familiar languages by analogy is still often useful, you stop relying on translation equivalents and exceptions to explain the new things you see.

Of course another funny effect is when there is L2 influence on the L3: for example, when I was taking a Swedish class, all of the students had already studied German, so we had, among other things, strange German accents (for no justifiable reason) in Swedish, as English speakers! So you never really move past using known languages as a base for learning others, but you can stop assuming that other languages should be like your native language plus exceptions.