Not to be diasgreeable composite 1, but I said "that level of backward compatibility"... is accurate, sure other brands have varying levels, including good levels, but none come even close to Pentax.
Pentax will fit and use every lens they ever made, but compared to Nikon, factor this:
if you need image stabilization, the Pentax will stabilize "Every Pentax ever made" Nikon, only the vr lenses have stabilization...
Pentax has had a 1.7 af teleconverter that will autofocus with "every Pentax lens ever made" Nikon, no such animal to my knowledge..
Pentax cameras can safely use every flash they ever made without frying the electronics from too high a sync voltage from the flash..
That is to say the system has it's limitations true, but if you are on a budget, it is worthy of consideration.
Now about old lenses, here is one thing I can attest to..
I have an old Pentax 50mm 1.4 M series lens, and a much newer 50mm 1.4 fa lens.
optically identical, but one is an old manual focus, no electronics manual aperture lens the other an autofocus etc..
On the newer lens matrix, center-weighted and spot metering and auto aperture set by the camera, or manually on the lens, are available but the older lens center weighted and spot are available and you set the aperture by ring on the lens body..not in camera..
the af lens takes about one turn of the af ring to go from min focus to infinity, the manual lens takes 1 and a half turns to do the same.
The manual lens is mush easier to control manual focus than the af version...
the older lens is the one I find better for video, by a huge margin... the feel and control and size of the manual focus ring are just plain better.
it is worth looking at/trying out, the old lenses if you're shooting video.