I'm really not interested in the opinion of an English major on the subject, unless they're also familiar with linguistics or they've learned a foreign language

I'm really not interested in the opinion of an English major on the subject, unless they're also familiar with linguistics or they've learned a foreign language to a high level. This is a heuristics issue monolingual English speakers are susceptible to. I spend most of my time with non-native English speakers (Italians, Germans, and Japanese people), and they all report that English is less complex than their native language. Of course English (and especially the mastery of English) is complex, but it's not exceptionally complex. Your "objective criteria" is sometimes not very objective, and sometimes not very good criteria for language complexity. 1. This point isn't objective. Borrowing grammar from German and French does not mean that we've accumulated more exceptions in our grammar than any other language, it doesn't mean that other languages have not also borrowed, and this does not objectively prove that English has more exceptions than any other language. Russian requires you to decline a noun by grammatical number (single/plural), one of six cases, and one of three genders. Even after you take these three axises into consideration and find the proper declension in your declension matrix there's still a chance that it doesn't follow the rules that you learned. You can either call this a system of 3 different noun types (one for each gender) with a ton of exceptions, or you can classify it as 18 different noun types with a ton of complex rules, and nearly no exceptions. English is like this as well (particularly phonetics, which is related to the next point). It's hard to be objective about language. 2. We agree on this point, but I want to point out that Chinese and Japanese are an order of magnitude more complex when it comes to phonetics. You need to learn several thousand characters to be considered literate as an adult. I can't read Chinese, but in Japanese the pronunciation of these characters changes depending on context. High frequency examples: 上 - 18th most common Kanji used on Wikipedia, 8 different pronunciations (9 if you consider ショウ and ジョウ different) 生 - 27th most common Kanji on Wikipedia, 12 different pronunciations 3. Lexical similarities isn't a relevant criteria. The inverse to your point, nobody would say "Russian isn't a complicated language because it's lexically similar to Ukrainian." Both can be complex if they're lexically similar, and a lexically unique language could be utterly uncomplicated. The most charitable interpretation I can speak to this point is that lexical similarities definitely help language acquisition, because you get a lot of words "for free" so to speak. 4. Again, I don't think vocab size is a great criteria for language complexity, and it's not very objective. We could inflate this number with a huge dictionary full of words that nobody uses. Some times words are "buy two get one free," like "wind, shield, windshield". Many languages have this (必ず without fail、勝つ victory、必勝 certain victory) and it doesn't reflect complexity in any consistent way.