The main technical difference is that in lowsec there are gate guns that sort-of help to protect you from campers. In nullsec people can also deploy warp bubbles that can prevent you from warping away, which changes the strategy a bit.
The real difference is that lowsec is full of trigger-happy yahoos that are just looking for action - and they will shoot at anything, even if it is probably bait.
Nullsec is much more organized, and most of the effort there is to amass huge amounts of money. They like to have overwhelming fleets so that they don’t lose any ships - because ships cost money. They also try to be allies with everybody, to avoid any ship losses. They have semi-automated intel channels so they can avoid PVP unless it is on their own terms.
There is also something called NPC null, which cannot be “owned” by anybody. It is like an extension of lowsec, but with warp bubbles and no gate guns.