There's morphological complexity and then there's semantic one. I grew up speaking one of Turkic and one of Slavic languages. Morphology-wise, both of those are massively more complicated as compared to English. But English is way more analytically powerful/flexible. It allows for constructions and frame of mind that are not quite possible in non-analytic languages. For even more dramatic example, consider math. The latter has barely any morphology to speak of. But its expressive flexibility is legendary if not unbeatable. Math famously allows for generalizations that simply do not exist in nature, just because reason/analysis allows for it.
It's also a matter of culture. For example, writing overly long sentences in passive voice in Russian is encouraged if not considered a mark of sophisticated writing whereas in English it's a massive faux-pas. Reading English I get the sense that meaning takes precedence over syntactic complexity. I can see why English might come off as, uhh, simple if one is used to bloated prose and reading between the lines. In math every word and sentence matters. I sense English strives for that as well.
Besides, English handles morphological complexity by way of syntax and massive semantic overloading of words.
Aside from logically vivid constructions and phrases, as a nonnonative speaker of English, I really like secondary/n-ary meaning to simple/common/"little" words that we are typically not taught as ESL (or EnglishAsNthLanguage). For example, I was delighted to discover "we made it" = "we are successful", "we made here on time" = "we got here on time", "we are making good time" = "we are traveling faster than expected" etc. I also love English constructions like "it's pretty ugly", "it's little too much" etc :) Basically, anything in English that doesn't have 1-1 equivalent in Slavic/Turkic languages immediately perks up my ears/eyes. Thankfully, there's a ton of that in English.