Lots of languages are hybridized from multiple, overlapping waves of conquerers, "but English's hybridity is high on the scale compared with most European languages," which gives us a realm of weird pronunciations, weirder spellings, inconsistent grammar, and a near-unique situation whereby speakers of languages that are close cousins to English can more-or-less understand English, too.
The amalgam of inconsistently blended Celtic, Norse, French and Latin make English a nightmare to learn, speak and spell — which makes the language's success in the world something of a miracle.
As long as the invaders got their meaning across, that was fine. But you can do that with a highly approximate rendition of a language – the legibility of the Frisian sentence you just read proves as much. So the Scandinavians did pretty much what we would expect: they spoke bad Old English. Their kids heard as much of that as they did real Old English. Life went on, and pretty soon their bad Old English was real English, and here we are today: the Scandies made English easier.
I should make a qualification here. In linguistics circles it's risky to call one language 'easier' than another one, for there is no single metric by which we can determine objective rankings. But even if there is no bright line between day and night, we'd never pretend there's no difference between life at 10am and life at 10pm. Likewise, some languages plainly jangle with more bells and whistles than others. If someone were told he had a year to get as good at either Russian or Hebrew as possible, and would lose a fingernail for every mistake he made during a three-minute test of his competence, only the masochist would choose Russian – unless he already happened to speak a language related to it. In that sense, English is 'easier' than other Germanic languages, and it's because of those Vikings.
Old English had the crazy genders we would expect of a good European language – but the Scandies didn't bother with those, and so now we have none. Chalk up one of English's weirdnesses. What's more, the Vikings mastered only that one shred of a once-lovely conjugation system: hence the lonely third‑person singular –s, hanging on like a dead bug on a windshield. Here and in other ways, they smoothed out the hard stuff.
Why is English so weirdly different from other languages? [John McWhorter/Aeon Essays]
(via Making Light)