How important is the gate threshold voltage on a MOSFET
The Gate Threshold Voltage VGS(th) refers to the voltage below which the FET stops conducting completely, defined by some manufacturers as the gate voltage at which the drain current crosses the threshold of 250μA.
It is not a maximum gate voltage (and nor is it the minimum gate voltage at which the FET can be used as a switch for useful amounts of current).
There are graphs in the datasheet in which you can look up what the FET does at various Vgs (the transfer characteristics, figure 6 in the datasheet), which is more interesting for designing. Figure 5 shows the I/V characteristics for various Vgs.
There is an Application Note here that explains it quite well.