I still cannot be sure how a new word'd be pronouced correctly just by looking at it.
Even native speakers can't, because we don't have very stable spelling rules, since it is, as mentioned, basically a hodge-podge of various languages.
A quick look at Word of the Day on dictionary.com gave me "paroxysm". Apart from having no idea what the meaning could be, I can think of three different pronunciations (PAIR OH ZY ZAM; PAH ROCKS ISM; PAR OX ISM), and various combinations of them. The actual pronounciation is close to "PAIR OCK SIZ UM".
Of course, this is nowhere near a common word, since I'd recognize those, but it illustrates my point.
Another good example is looking at the words "Tough", "Though", "Thought", "Through" (and "Throw"), "Thorough". Just looking at them, you realize it's somewhat silly.
Also take a look at http://www.hep.wisc.edu/~jnb/charivarius.html
; the subject is slightly different, but it's wonderful.
In standard American English, there is no difference in pronunciation between your and you're, so using your for you're is simply bad spelling.
I'm going to throw my two cents into this hat-- they have two very different pronunciations to me. "your" is pronounced like "yoor", whereas "you're" is more like "yer". Although I think it's more a matter of people pronouncing your as yer that results in this.
I remember reading an "accent" test a long time ago that would ask questions like that-- whether there was a different pronunciation for various words (like, there and they're, stuff like that), and it was fairly interesting.
For those that care, I've spent most of my life in the midwestern united states, but I've been told I have closer to a "Boston" accent. I personally think this is because of my hearing problems as a kid (I used to get so many ear infections I looked forward
to them because I liked the way the medicine tasted), and nothing special.
Anyway, annoying as it is, the written system greatly helps my English skills, in my opinion. Hearing a word that doesn't sound quite right, and mentally linking it to a logical spelling, and thus deriving the word intended is a great way to get over accents and such (although it does occasionally give me headaches when I listen to British TV) where I otherwise would've had to guess the word from context.
I'd like it if English moved to a syllabary, true, but it would take an extremely massive revolution for something like that to happen, and it would be a very large and complex system to be able to keep the sounds English has now intact (and killing off pronunciations would make the switch even harder). Hell, it would almost be easier to get people to switch to an entirely new language, and then get that to replace English.