Is there any reason why so many MOSFET datasheets have negative parameters (Vgs, Vds, etc.)? Why are they negative, not positive, and does that fact they are negative have any impact on it's application?

 

Often PFETs use negative numbers while NFETs use positive. It denotes how they are used. Ie drain to source voltage.

 

A MOSFET might also be exposed to negative voltages in normal use, and it's useful to know what they can tolerate. For instance I have a 12V circuit where I use an N-mosfet for the high side switch. The gate voltage is bootstrapped up to 24V so that I can turn on that high-side MOSFET. When the circuit takes the gate low, it goes to -12V relative to its source pin. The data sheet says the MOSFET is rated to -15V, telling me this is OK.