> English happens to be radically unlike most Indo-European languages in precisely the ways that make it fairly similar to Mandarin.

I don't really view English as radically different. I just view it as more fluid. Mandarin I only know of peripherally. French I know some and German I know quite well. English really is very similar to German. Old English and Altdeutsch (Old German) were almost the same language (800-1200 years ago) but when French became the court language of England (after 1066), there was no central authority maintaining what common English was so it evolved into something very different with Middle English (which is almost recognizable as Modern English) by, say, the Tudor dynasty.

Linguistically speaking this was a very interesting phenomenon and one that I don't think has happened too often elsewhere (where the language of the aristocracy wasn't the language of the people and was essentially imported). It led to what I call the democratization of English.

I find it really interesting that the lack of a central authority imposing standards and maintaining the "purity" of the language actually led to it becoming much simpler.

> ... given that Mandarin (for example) has ...

Like I said, I'm no expert at Mandarin. A friend of mine has lived in Taiwan for years and learned Mandarin and he tells me a lot about it. One thing Mandarin has that English doesn't is formal tone (rising, rising-falling, etc). This can radically change the meaning. This is a mechanism for communication that native Mandarin speakers just can't find an analog to when they learn English.

> All languages have the same amount of grammar

While I think we'll both agree that certain languages are easier to learn for the speakers of certain languages than others (eg Spanish speakers learn Italian far easier than Mandarin, Mandarin speakers will learn Vietnamese far easier than Spanish) I disagree with your waterbed analogy, implying there's some sort of conservation of language complexity in play.

I believe that some languages are fundamentally easier to learn than others when you account for starting biases and also that certain languages convey information better and more easily than others.

Literacy is one measure of this. In 1929 (IIRC), Turkey's president switched their alphabet from Arabic to a highly phonetic Latin alphabet. Literacy rates shot up and someone who was previously illiterate could learn to read the new Turkish in ~6 months.

Compare that with Taiwan where they have competitions in high school at how fast you can find words in the dictionary.

Also, as a speaker of English, if I'm talking to you on the phone, I can tell you a new word and how to spell it. There is no equivalent for Mandarin (with a new character).

My argument is there is a "cognitive price" to be paid for arbitrary complexity in a language as well as not front-weighting of information by importance.

An example of the latter: separable verbs in German. (jdm/etw) bringen = "to bring". (jdm) umbringen = "to kill" so:

Ich bringe meine Frau (I bring my wife) and:
Ich habe meine Frau gebracht (I brought my wife) but:

Ich bringe meine Frau um (I kill my wife) and
Ich habe meine Frau umgebracht (I killed my wife)

Likewise, the arbitrary rules about the agreement of number, gender, case and article (eg German can change depending on whether you're saying "a student" (indefinite article) vs "the student" (definite article) all other factors being equal).