I think, as others have mentioned, build quality is a perception that seems to be linked to heft and the apparent quality of the materials used. So by that measure, a Bessa R, say, doesn't stack up to an MP. But until the item in question has been in service for some time, it's hard to evaluate whether indeed it's of good build quality or not. It's not an initial impression, since it has to apply to how well the item functions over time. It is linked to reliability -- a beautifully made item that doesn't work reliably can't be said to be of good build quality, can it? It's pretty hard to say that a Toyota Corolla or Honda Civic, two of the most reliable vehicles in existence, is of lesser build quality than a much pricier BMW 5 series.
That Bessa R may begin to have its paint rub off relatively quickly, and it feels plasticky. But does it keep working without the need for servicing? That's the true test. I hate to use a gun analogy, but I remember during the Vietnam War much was made of the fact that the Colt M-16 (built here in Hartford) had trouble dealing with mud, moisture and other conditions experienced out in the rice paddies. Meanwhile, the supposedly shoddy AK47 had no such problems, and was more durable. So which had the better build quality?