The speaker listed 5 things about English that make it more difficult to master than a lot of other popular languages. (She actually said "uniquely difficult", but I think that's probably overstating the case by quite a bit. English may have some unusual phonemes, for example, but I think those are a lot easier for people to learn than features like tones and vowel length are for people who don't have them in their native language. And English grammar is not particularly exotic.) Anyway, I thought I'd share them.
The first one is that the English lexicon is huge. There are more than a million words in English, which is a lot more than most (all?) other languages. ("How many words are there in Language X" is actually a very difficult question to answer, because it depends on how you do your counting. But regardless of exactly what you count as a word, it's clear that English has a lot of them.)
Her second two points are that it has a lot more vowels and consonants than it does letters, which I will refactor into two more distinct points: English has a lot of phonemes, and it has a very difficult orthography.
There are 15 to 17 (or maybe 19 or 23, again depending on who's counting and who's speaking) distinct vowels in English. We represent them with only 5 letters. So it's much harder to sound things out than in, say, Romance languages. It's also a lot more vowels sounds total than most languages have.
And then we've got 24 to 29 consonants and only 21 letters to write them. I think that's on the high end. It's certainly not as many as Hindi/Urdu (31-37 consonants) but it's a lot more than Hawaiian (8). And they can cluster up to three at a time (e.g., "strengths") on either side of a vowel. So in terms of number of legal combinations of sounds, there are a hell of a lot of them.
And of course, English spelling is notoriously difficult. Not only do we not have enough letters, and not only did a lot of spellings get fossilized just as pronunciations were in the middle of shifting, English spelling doesn't just denote sound, it also carries a lot of etymological information. English speakers are an acquisitive lot and delight in picking up shiny new loanwords from other languages and transplanting them, spelling and all if possible, into their own. This is why spelling reform is never going to catch on: because "tion" means something very distinct from "shun", even though they're pronounced the same, and spelling them differently makes it easier to infer the meaning of words you may not have seen before.
This dovetails neatly with the next point, which is that English has a lot of bound morphemes, syllabic elements that carry meaning but don't stand as words on their own. At least, that's how I interpret what she actually said, which was "some syllables have meaning on their own". It seemed strange that that would be a rare feature of a language, since lots of languages have all kinds of suffixes and prefixes and infixes, but if we're talking about things like Greek and Latin word roots that have been incorporated and recycled in other words, yeah, that's a very English-y thing.
Finally, there's the fact that English has a lot of unusual phonemes. "Th", in particular, both voiced and unvoiced? Yeah, that's a weirdo sound that nobody else uses. Except, like, Icelandic speakers. Also uncommon are having both 'b' and 'v' as distinct sounds and (especially hard for non-native speakers) distinguishing between 'l' and 'r'. Lots of languages don't do that, it turns out.
Personally, I think a lot of these things make English interesting and fun, and that a weird and complicated language with a lot of expressive power is much cooler than the kind of straightforward and logical system that most of the conlang crowd is always trying to design.
But I freely admit that I'm a freak.