What is the most interesting feature you've seen in a language?

The geographic positioning of languages like Guugu Yimithirr is fascinating. You wouldn't say "can you move left a bit", you'd say "can you move east a little". There's some evidence to suggest the Guugu Yimithirr are rather better at direction finding than people who speak other languages. It's not exclusive to them either, a number of Polynesian languages do that too.

My favourite though is that of the Matses people of Peru. They have suffixes which denote evidentiality, so you can't just say "A leopard was here", you have to say "I personally saw a leopard here" or "I heard from someone that a leopard was here" or "I think a leopard was here but it's just conjecture"

I don't know if this really qualifies since it's not, strictly speaking, "a language" but seeing as nobody else has responded I figured I would throw this out there in the hopes to spark some further language related discussion:

One of my favorite systems of writing, the Canadian Aboriginal Syllabics, is somewhat unique in that it is a modern construction and is not used for any one particular language but instead (similar to Latin script) has been designed/adapted to apply to many Aboriginal, Indigenous and Native languages in Canada that did not have their own systems of writing, many of which are completely unrelated and not mutually intelligible to one another.

The way it accomplishes this is by using unique static shapes and rotations of those shapes to represent common vowel sounds and consonants-vowel pairs, as well as diacritic marks before/after/on those symbols to indicate additional consonants, consonant clusters, vowel length, etc. This makes it highly adaptable (even able to easily incorporate loan words from other languages) and also remarkably easy to learn. And it also has full Unicode support as well:

https://en.wikipedia.org/wiki/Unified_Canadian_Aboriginal_Syllabics_(Unicode_block)
https://en.wikipedia.org/wiki/Unified_Canadian_Aboriginal_Syllabics_Extended

So I can even show some examples of how the system works:

ᐃ = i, ᐱ = pi, ᑎ = ti, ᑏ = tii, ᑍ = taai
ᐊ = a, ᐸ = pa, ᑕ = ta, ᑖ = taa
ᐅ = o, ᐳ = po, ᑐ = to, ᑑ = too
ᐁ = e, ᐯ = pe, ᑌ = te,   

And so if a syllabics user needed to write the English loan word "potato" (po-taai-to), they could do so with ᐳᑍᑐ and here is an example of real world use of the language.

Not that interested, but in French you can use "si" to respond to a negative question in the negative. It's a nice way of resolving an ambiguity that sometimes arises in English. Example:

"Tu ne m'aimes pas?" (You don't like me?)

• "Oui." (Yes, I don't like you.)

• "Si!" (Yes, I do like you!)

Maybe not quite a feature. It's interesting to see what words are omitted from a language. Without a word for something, it's hard to conceptualize an idea. I've been told that the Lakota Sioux had no word for stealing. When Europeans first talked with them they found this odd. When they tried to explain the concept ("What if I took this from you and didn't return it?") their response was confusion. Why would someone do that? And in English we have a lot of different words for stealing, each with slightly different definitions.

I don't really have a source for this and I'm having difficulty finding one. I heard this from an English professor a couple of months ago. Interestingly, Google mentions the Lakota's stealing of others' horses as trophies. If what I was told by my professor is true, maybe they considered this kind of theft different.

In Esperanto, there are progressive, perfect, and prospective compound tenses.

https://en.wikipedia.org/wiki/Esperanto_grammar#Compound_tense

This allows you to make semi-complex statements such as "I was going to speak" flow like "Mi parolontis" (or more explicitly "Mi estis parolonta").

I actually find Esperanto so interesting because of the fact that it doesn't have that many interesting features. Everything makes sense and is what you would expect to need in a language.

Ah you kinda picked mine! I love how much info you can give in Japanese like this. My favourite, the different that/there you use when you know the person you're talking to doesn't know what you're talking about verses when you assume they do.

I also love that there is no assumption of gender when speaking (and generally writing too, though you can specify) in Cantonese and Mandarin.

I was interested in constructed languages (conlangs) for a time, and I came across this article: "Designing an Artificial Language: Arabic Morphology". Words are generally made up of a base of consonants plus templates of prefixes, infixes, and suffixes that denote the specific meaning. Coming from English, this system seemed really cool. (It also struck me as a good way to make lots of new words for a conlang easily, so if I were to ever get back into that I'd probably use something based on it.)

More info, explained better than I can: "Arabic Morphology Introduction" and "Arabic Templates".