In a sense it wasn't the AK-47 in itself that was such a big deal. After all there were other assault rifles around at the same time. The 'big' thing about it was its proiferation. Large numbers of AKs and varients have been exported through the 20th Century, probably more so than any of the AK-47's initial competitors. One thing to keep in mind is that while the AK as easy to make, this is a relative thing. Making a 'proper' AK is difficult to do without an established industrial base (parts of the AK's mechanism for example are chrome plated, which isn't really something you want to do on a small scale). So export and mass production of the AK also followed the proliferation of the means to produce it (the specific tools and the general economic means) and perhaps to an extent this also links to the 'proliferation' of socialist economic planning and the drive for industrialisation you see in the post-colonial era. For these newly industrialising countries, the AK was easier to produce comparitively and best of all it was available. Proliferation in turn makes the weapon relatively cheap (since it would already be widely available), creating a bit of a spiral. You might like to have a read of C.J Chiver's book The Gun, which is sort of a socio-political history of the AK (with notable appearances by other automatic weapons from history like the Maxim gun and M-16), which particularly looks at the production element of the AKs history.

I guess what I'm trying to say is that the relatively rapid proliferation of an assault rifle around the world does have game changing results. Giving more firepower to the infantryman etc. But it wasn't really the AK's design that did this as such, but the circumstances.

A quality of the AK design did have one important factor - its ease of maintenance and durability. Not only were a lot of AKs sold/built, they're much more likely to still be around and basically functional. If you're not fused by quality, an AK can probably be maintained or repaired with a fairly basic workshop.