Personally, because Greek is an Indoeuropean language and its my mother tongue, I had not problem learning English when me and my family were living in Philadelphia but I don't have that much difficulty with other Indo-European languages either. I find Turkish, on the other hand, hard to learn. Therefore I think its easier to learn a language that is closer to ur L1...

There's no question that it's easier to learn a closely-related language than an entirely different one. The cases of interference are trivial compared to the massive advantages in terms of parallel syntax and, especially, vocabulary.

If I need to say something in, say, Spanish, I can always guess at it based on my knowledge of English. Sometimes I'm wrong and that word or construction doesn't exist (or exists with a very different meaning), but more often than not my meaning still makes it across. This doesn't work at all with Chinese or Turkish. Either I know the write thing to say or I'm completely SOL.

This seems to affect me quite a bit, especially with Celtic languages. While they are IE too, they are just so weird that my brain doesn't like learning them. :/

On the other hand Japanese seems pretty easy, except the kanji, although they aren't hard; you just need a long time to learn them.

Also, Inuktitut has no reason to be even slightly irregular.

With Japanese, you have to drop anything you think you know about basic syntax. It's totally alien. That's my biggest issue.

I recently read that Japanese speakers tend to pick up Turkish fairly easily. Guy Deutcher, commenting on this, suggests this may be a function that both languages are SVO with postpositions, which tends to drive word order in parallel.

Based on this, I would suggest that of distant languages, similarity in syntax will be a significant factor. I did find that Mandarin, being SVO and isolating, was not as difficult as I expected.

Actually they are both SOV which is a common feature of Altaic languages (not implying that Japanese is one, it's just in the general Sprachraum of them). But you are right about that. They are also both agglutinative languages.

I agree with ILuvEire that Japanese is much harder than it looks. It has very little in common with English, structurally speaking.

I'm Cherokee, and while I'm not a native speaker, my grandmother was and she taught me. Cherokee is truly alien however. There are no /p/'s /b/'s, /v/'s or /f/'s. The lips do not move when speaking Cherokee. If you speak it with a grin, you're close

And don't forget "the kludge of heroic dimensions"—applying Chinese characters to a language that didn't really need them.

Yes, that was a typo.

Not necessarily. Kanji can be learned in a matter of months, if you use the proper method.