I started off by posting a series of comments scattered all over the page, but I thought I should sum them up in a standalone answer.
Generally speaking, there have been similar shifts in many other languages. And they are even happening right now as we speak.
But first things first. Since you mentioned Slavic languages, as the most obvious example, in Old Church Slavonic, an o was an o. In contemporary Russian, it can be anything from a schwa to an ʌ to an ɔ, depending on the position relative to the stressed syllable (e.g. молоко, milk, /məɫɐˈko/ or /məlʌˈkɔ/; водоворот, swirl, /vədəvʌˈro̞t/). Also, in Old Church Slavonic, there were a number of nasal sounds, which are absent in pretty much all contemporary Slavic languages with the notable exception of Polish.
Secondly, don't get me started on German. If you don't know how to correctly pronounce Soest, Troisdorf, Huonker, Pankow, Laermann, Hueck, you will pronounce them wrong. It happens to native German speakers all the time.
Speaking of Germanic languages, the most notable vowel shifts happened in German and Dutch (Wikipedia even mentions them in the article on the Great Vowel Shift). It's just that there was at least some concerted effort to keep the spelling consistent with the (changing) pronunciation. The pronunciation shifts were accompanied by spelling shifts, if you will. Hence the popular but wrong assumption that there weren't pronunciation shifts to begin with.
In other words, what made the pronunciation stray so far from spelling in English was not the Great Vowel Shift; it was the absence of the accompanying Great Spelling Update.
Now, it's always a bit harder to explain the absence of something rather than its presence, though one of the other answers does provide an interesting link. On a more general note, I will say that spelling reforms are the domain of politicians, one of the most prominent and recent examples being the German orthography reform of 1996, kicked off by the Conference of Ministers of Culture and later monitored by the International Commission for German Orthography. English, however, traditionally lacks such regulatory bodies.
Anyhow, vowel shifts happen all the time, especially on the dialect level. Now that I have zeroed in on German, I'll just take Bavarian as an example. In Bavarian, viel is not pronounced as /fiːl/ and ein Haufen is not pronounced /aɪ̯n ˈhaʊ̯fm̩/. But again, there is some effort to keep the spelling consistent with the pronunciation, so if you came up with the crazy idea to write a Wikipedia in Bavarian, you would spell viel as vui, vei, vii or fui, and ein Haufen as a Haufa, to reflect the actual pronunciation. And there are also Wikipedias in Ripuarian, Plattdeutsch, Alemannic... It's hard to imagine, say, standalone Australian, Canadian or Texan Wikipedias where the spelling mirrors the local dialect in such a manner.
I guess I can sum these ramblings up as follows: vowel shifts happen all the time. Spelling conventions are a question of politics and culture.
I've done nothing but sit on my rear all day trying to find you an appropriate answer. I've only come across one article online that seems to collectively dictate anything and everything that I've being reading. It seems that Canada defines the majority of its culture upon its language (and spelling).
While I have to agree with Robin Michael that you'll never really find yourself with a simple answer, I do hope this article may shed some more light.
According to him, the Canadian language, the culture, is being slowly diminished by your friendly neighbors to the south of you. (Hello!)
The Canadian culture was to be unique and different but outside factors have created mass confusion on how to spell.
(There were many other websites that mentioned bits and pieces of what this article says. I chose to link you this certain article, alone, because it was the only article to contain a bit more information closer to what you're asking.)
I posted this too soon it seems. I found this article that seems to have a somewhat similar take to the other article, but with a semi-different spin.
Taking your point to heart, I decided to refine my search, looking strictly for only books. Finding this book in particular, the Google Preview looked promising. There is a lot to read, however. I don't think you'll mind that though; you seem to really want this answer!
Soft edit: I normally dislike Wikipedia, and I don't know if you saw this or not, but I think it gives a general clue as to how Canadian English became mixed. Link
Best Answer
Direct from Greek ῥυθμός (rhythmos). "rh" is not an unusual word start in Greek, "y" is just a vowel, "-os" turned into "-us" in Latin then fell off when accepted into English, so the vowel that would have been in the syllable with "m" went away.