For some reason, people around the world are learning English to a degree unprecedented in history, to a degree that the number of English second-language speakers now exceeds the number of native speakers. Thanks to the worldspanning influence of the British Empire and the United States, and with major English-speaking countries found on almost every continent, English has become a lingua franca to a degree that French could only have dreamt of back in the day. Hundreds of millions of people are learning English... you almost have to wonder if they're gluttons for punishment.
Yes, yes, I know the value of English, particularly in terms of its widespread nature meaning that people from diverse areas are likely now to have it in common. But that doesn't change the fact that it is English, this ramshackle, kludged-together contraption of nouns and adverbs that hangs together with spit and prayer and bailing wire. Tens of millions of native speakers don't appreciate this, because the flipside of English's new universality is that it gives native speakers a reason to remain unilingual - why go to all the effort of twisting your brain around some other language, when so many of its speakers are likely as not already learning yours? This is a significant problem, especially in North America - English Canada's relations with Quebec and the United States' positions on Mexican immigration might not be as fraught if more of the majority spoke the language of the minority.
Still, learning another language doesn't solely involve maneuvering your mind to pick up the ins and outs of how it works; the same process happens with your own, giving you the opportunity to see your native language from more of an outsider's perspective.
I've recently renewed my attempts to learn Japanese through language exchange as well as textbooks, which means that I use my shaky grasp of my native language to help someone else learn it, and they in turn help me out with their native Japanese. So far, one of the greatest attractions I've found with Japanese is its simplicity in comparison to the Romance languages; there are only two tenses, many things don't have to be stated once it's obvious what you're talking about, and words aren't gendered! Not like in languages like French, where you have to remember whether or not a desk or a pencil is masculine or feminine. Even when the class had a talking pineapple on VHS to help us out, trying to get down the whole je/tu, il/elle, nous/vous, ils/elles thing in French represented a huge barrier of entry to me in my elementary and high school classes.
English's weirdnesses aren't necessarily as obvious. I certainly would never have thought about them if it hadn't fallen to me to explain them. For example, take the following sentences: "While hiking in the woods I encountered a bear" versus "I encountered a bear while hiking in the woods."
My Japanese exchange partner expressed confusion that these two sentences had exactly the same meaning, to the extent that I had to ask around at work the next day to make sure they did have the same meaning, and I wasn't just moving bad freight. I couldn't even explain at first why they were the same! These are things that a native speaker doesn't need to consciously know; after being brought up in the language, surrounded by it, native speakers understand and speak it less through education than by a sort of socially-inculcated instinct, no matter what language it is - I would imagine mostly, though, for people who are brought up unilingual.
Beyond that, there's English's array of modifiers, "on" and "by" and "of" and "with" and so on, used in differing ways from sentence to sentence and generally only known to be right by native speakers because it sounds right. Confronting something like that head-on forces you to appraise it in an entirely new way. With another native speaker, you might just say "don't talk like that, it's wrong" - when it comes to a learner, that doesn't go far enough. You have to know the rules before you can start breaking them with confidence.
The English language, as it stands today, is a premier example of evolution in action - memetic evolution, certainly, but many of the same factors still apply. English is not even close to being an ideal language - the fact that that word isn't spelled "langwij" is just one piece of evidence for that - just like the human body is not even close to being ideal for a species of thinking, industrial beings like ourselves. It doesn't need to be ideal; it just needs to work sufficiently well, just like the reverse wiring in the human eye. The English language is a memeplex that has evolved piecemeal over a thousand years and more, incorporating bits and pieces from here and there, working well enough to be understood and lucky enough to be piggybacking on peoples that bent the world to their will.
That doesn't change the fact that, also like humanity, it's a miracle that English managed to get anywhere at all.