English...

I was reading 'Eats Shoots and leaves' a few months ago and was really intriguied at the manner in which the punctuations in the English language have come up to this day's stature. A misuse of a small mark (read punctuation) makes the meaning twist and turn 180 degrees. I quite enjoy the following the gender based example.
On the other hand, the English language as such has gone a drastic metamorphosis in the way the words are used. In one of my Sanskrit classes, the guruji (teacher) usually mentions the evolution of the language over time. It seems that none of the words have undergone a change in their meaning over the long period it has been in use. He usually quotes the example of how words in English have changed in their primary meaning over the years. The most common word he uses as an example is gay.
Similarly, I was quite amused the see a similar example when reading 'An Autobiography of a Yogi' with respect to the word ejaculation.
This brings me to wonder what makes the primary meaning / usage of a word change. Is it the media that plays an important part? Is it the emphasis of technology or science? Any thoughts on how these changes get slowly into the act?  Wikipedia has some details on this semantic change.