Understanding Vgs(th) on a datasheet
Even though you've picked an answer, I note that it doesn't really answer your question about Vgs(th). (Sorry, Andy.)

Note that on the data sheet, Vgs(th) is specified at Vds = Vgs, and the current level is 250 uA.

In this case, the maximum is simply the maximum required to produce 250 uA. If you give the gate less voltage, you may get less current (or maybe not, depending on the unit). It's possible that you might get lucky as low as 0.65 volts (the minimum).

So in this case, if you like, "maximum" sort of means "minimum", in the sense that it is the minimum required to guarantee 250 uA. This is entirely different from the maximum limits found elsewhere, which define what you can do without potentially damaging the device.