Some differences are:
IPSC
Make Ready - This command is given for an unloaded started. You may check your dot and if you point the pistol to within a 3' radius of where you are standing, you will not have an issue. If you rack the gun and drop the hammer with the trigger, you will be given a warning for the first occurrence and subsequent times you will issued a PE. This is noted on your scorecard. They will say something if you try and sequence the targets with your firearm.
Load and Make Ready - This command is given for any start where there a round in the chamber. Again you may check your dot within the 3' circle but ONLY with a unloaded pistol.
During the walkthrough, you are not allowed to have anything in your hand(s) to point at the targets with. Not a mag, coffee, towel, nothing. This might be considered to be over the top, but different ROs will call different things, don't put your fate in the hands of an official.
Unless specifically restricted, the bounds of what is considered safe handling of a pistol reload or otherwise is 90 degrees in all directions (180) in USPSA. If there is a restriction like not pointing over a berm during a reload it will be stated. I have seen this in Europe but not North America.
I think those are the main things that can cause you grief at least in Open.
Good Luck!
R&G