English is a great language, it really is.
It (due to borrowing liberally from other languages) has a really wide range of words.
Take words meaning fast, for instance. We have: fast, rapidly, quickly, hastily, swiftly, hurriedly, snappily, and probably many others.
Yes, I'll agree that the spelling is a little inconsistent since the Great Vowel Shift.
But it doesn't have useless genders (French, German), or complex grammar (Polish, Russian, Latin), and there's so much English content around, so the bar is fairly low for being able to learn it. Yes, it might be hard to master all the phrasal verbs, and strange rules (fewer/less?), but to get up and running isn't too tricky.
However, there's one sound in English that a lot of non-native speakers have problems with and that is the "th" sound.
The th sound
The French say "zis", the Dutch say "dis", and various other countries have ways of saying the "th".
(For the record, you need to stick your tongue out a little between closed teeth, and as you say "th", pull your tongue in. I guess it feels odd to a non-native speaker to have your tongue flicking in and out of your mouth like a snake when speaking, but it's the only way to properly say th.)
But there isn't a single th sound in English - there are in fact two.
"This" and "thing" for instance have a slightly different th - one is voiced, and one is unvoiced.
If you touch your voicebox when saying "this", you'll feel that it's vibrating (voiced) - which doesn't happen when saying thunder.
As Wikipedia puts it: the voiced dental fricative /ð/ (as in this) and the voiceless dental fricative /θ/ (thing)
ð is Eth, and θ is Theta.
While I waiting for a suite of tests to run, I started reading up about other things, and it took me into areas I found interesting.
In some areas of the UK, people are starting to say th as v or f. For instance, brother as bruvva, as illustrated in this clip of Human Traffic, and birthday as birfday. I'm never really sure if the people doing it are aware they're doing it.
Another change I've heard over time is the dropping of the "yoo" sound in various words.
My Gran would say "I'll syoo him" for "I'll sue him". To me that sounds old-fashioned, and weird, but recently I've heard "noos" readers talking about "consoomers", which sounds very odd.
Maybe in a couple of decades saying nyoos and consyoomers will sounds as old fashioned as syoo does to me now?
American English says "stoopid" rather than our "styoopid" - perhaps this is from the influence of Spanish speakers converting estupido into stoopid?
Changes in pronunciation
The final thing that amazed me was the way sounds in English have changed.
First, there was the Great Vowel shift.
Before that, bite would be pronounced as "beet".
But there are lots of other sounds that have merged together.
These take place in different areas at different times, so not all may have occurred in the version of English you speak.
A fairly obvious one is the way New Zealanders say "e"s as "i"s - some examples in this - listen for family "mimbers", and "frinds".
Phonological history of English consonants
I've always pronounced whine and wine identically. But thinking back, my Gran pronounced these differently. The "wh" sound was always produced blowing air out, so it was more of a hwhine. Again, today that would sound out-of-date, so I guess this shows how the pronunciation changes.