I guess an interesting point is that the (AC linked) differential pair has a tanh input amplitude versus gain curve. The requirement that the transistors be matched in the usual (DC linked) long tail pair is a DC biasing only requirement. The two transistors in the AC linked version can be very different and still give symmetrical behavior. Anyway the slightly compressive tanh response allows for a smooth transition into oscillation with increasing positive feedback. A lot of circuits on the internet use FET's for regeneration. There you have to hope that you can bias the FET into a region of its operating curve where the gain it provides is compressive (decreasesslightly with increasing input amplitude). Even the well know designs by Charles Kitchin in QEX do not address that problem and are kind of hit and miss.
